diff --git a/llvm/docs/CommandGuide/llvm-nm.rst b/llvm/docs/CommandGuide/llvm-nm.rst
--- a/llvm/docs/CommandGuide/llvm-nm.rst
+++ b/llvm/docs/CommandGuide/llvm-nm.rst
@@ -141,6 +141,14 @@
    any
          Process all the supported object files.
 
+   In AIX OS, the default is to process 32-bit object files (ignore 64-bit objects). The mode can also
+  be set with the OBJECT_MODE environment variable. For example, OBJECT_MODE=64 causes ar to
+  process any 64-bit objects and ignore 32-bit objects. The -X flag overrides the OBJECT_MODE
+  variable.
+
+   In no AIX OS, default is to process all the supported object files. And do not support the
+  OBJECT_MODE environment variable.
+
 .. option:: --debug-syms, -a
 
  Show all symbols, even those usually suppressed.
diff --git a/llvm/test/tools/llvm-nm/option-X-AIX.test b/llvm/test/tools/llvm-nm/option-X-AIX.test
new file mode 100644
--- /dev/null
+++ b/llvm/test/tools/llvm-nm/option-X-AIX.test
@@ -0,0 +1,92 @@
+# REQUIRES: system-aix
+## Test default "-X" option and environment variable "OBJECT_MODE" on AIX OS.
+
+# RUN: llvm-as -o %t32.bc %p/Inputs/bitcode-sym32.ll
+# RUN: llvm-as -o %t64.bc %p/Inputs/bitcode-sym64.ll
+
+## Test default "-X" option.
+# RUN: unset OBJECT_MODE
+# RUN: llvm-nm --format=just-symbols %t32.bc %t64.bc | \
+# RUN:   FileCheck %s -DFILE1=%t32.bc --check-prefixes=BITCODE32 --implicit-check-not={{.}}
+
+## Test environment variable "OBJECT_MODE".
+# RUN: env OBJECT_MODE=32 llvm-nm --format=just-symbols %t32.bc %t64.bc | \
+# RUN:   FileCheck %s -DFILE1=%t32.bc --check-prefixes=BITCODE32 --implicit-check-not={{.}}
+# RUN: env OBJECT_MODE=64 llvm-nm --format=just-symbols %t32.bc %t64.bc | \
+# RUN:   FileCheck %s -DFILE2=%t64.bc --check-prefixes=BITCODE64 --implicit-check-not={{.}}
+# RUN: env OBJECT_MODE=32_64 llvm-nm --format=just-symbols %t32.bc %t64.bc | \
+# RUN:   FileCheck %s -DFILE1=%t32.bc -DFILE2=%t64.bc --check-prefixes=BITCODE32,BITCODE64
+# RUN: env OBJECT_MODE=any llvm-nm --format=just-symbols %t32.bc %t64.bc | \
+# RUN:   FileCheck %s -DFILE1=%t32.bc -DFILE2=%t64.bc --check-prefixes=BITCODE32,BITCODE64
+
+## Test -X option override environment variable "OBJECT_MODE".
+# RUN: env OBJECT_MODE=any llvm-nm -X32 --format=just-symbols %t32.bc %t64.bc | \
+# RUN:   FileCheck %s -DFILE1=%t32.bc --check-prefixes=BITCODE32 --implicit-check-not={{.}}
+# RUN: env OBJECT_MODE=any llvm-nm -X64 --format=just-symbols %t32.bc %t64.bc | \
+# RUN:   FileCheck %s -DFILE2=%t64.bc --check-prefixes=BITCODE64 --implicit-check-not={{.}}
+# RUN: env OBJECT_MODE=any llvm-nm -X32_64 --format=just-symbols %t32.bc %t64.bc | \
+# RUN:   FileCheck %s -DFILE1=%t32.bc -DFILE2=%t64.bc --check-prefixes=BITCODE32,BITCODE64
+# RUN: env OBJECT_MODE=any llvm-nm -Xany --format=just-symbols %t32.bc %t64.bc | \
+# RUN:   FileCheck %s -DFILE1=%t32.bc -DFILE2=%t64.bc --check-prefixes=BITCODE32,BITCODE64
+
+# BITCODE32:      [[FILE1]]:
+# BITCODE32-NEXT: C32
+# BITCODE32-NEXT: foo32
+# BITCODE32-NEXT: undef_var32
+
+# BITCODE64:      [[FILE2]]:
+# BITCODE64-NEXT: C64
+# BITCODE64-NEXT: bar64
+# BITCODE64-NEXT: static_var64
+
+# RUN: yaml2obj -DFLAG=0x1DF %s -o %t_xcoff32.o
+# RUN: yaml2obj -DFLAG=0x1F7 %s -o %t_xcoff64.o
+# RUN: rm -rf %t.a
+# RUN: env OBJECT_MODE=any llvm-ar -q -c %t.a %t_xcoff32.o %t_xcoff64.o
+
+# RUN: llvm-nm --format=just-symbols %t_xcoff32.o | \
+# RUN:   FileCheck --check-prefixes=XCOFF32 %s --implicit-check-not={{.}}
+# RUN: env OBJECT_MODE=32 llvm-nm --format=just-symbols %t_xcoff32.o | \
+# RUN:   FileCheck --check-prefixes=XCOFF32 %s --implicit-check-not={{.}}
+# RUN: llvm-nm --format=just-symbols %t.a | \
+# RUN:   FileCheck --check-prefixes=ARC32,XCOFF32 %s -DFILE=%basename_t --implicit-check-not={{.}}
+# RUN: env OBJECT_MODE=32 llvm-nm --format=just-symbols %t.a | \
+# RUN:   FileCheck --check-prefixes=ARC32,XCOFF32 %s -DFILE=%basename_t --implicit-check-not={{.}}
+
+# ARC32:      [[FILE]].tmp_xcoff32.o:
+# XCOFF32:      var_0x1DF
+
+# RUN: env OBJECT_MODE=64 llvm-nm --format=just-symbols %t_xcoff64.o | \
+# RUN:   FileCheck --check-prefixes=XCOFF64 %s --implicit-check-not={{.}}
+# RUN: env OBJECT_MODE=64 llvm-nm --format=just-symbols %t.a | \
+# RUN:   FileCheck --check-prefixes=ARC64,XCOFF64 -DFILE=%basename_t %s --implicit-check-not={{.}}
+
+# ARC64:       [[FILE]].tmp_xcoff64.o:
+# XCOFF64:       var_0x1F7
+
+# RUN: env OBJECT_MODE=32_64 llvm-nm --format=just-symbols %t_xcoff32.o %t_xcoff64.o | \
+# RUN:   FileCheck --check-prefixes=BOTH %s -DFILE32=%t_xcoff32.o -DFILE64=%t_xcoff64.o --match-full-lines
+# RUN: env OBJECT_MODE=any llvm-nm --format=just-symbols %t_xcoff32.o %t_xcoff64.o | \
+# RUN:   FileCheck --check-prefixes=BOTH %s -DFILE32=%t_xcoff32.o -DFILE64=%t_xcoff64.o --match-full-lines
+
+# BOTH:        [[FILE32]]:
+# BOTH-NEXT:   var_0x1DF
+# BOTH-EMPTY:
+# BOTH-NEXT:   [[FILE64]]:
+# BOTH-NEXT:   var_0x1F7
+
+# RUN: env OBJECT_MODE=32_64 llvm-nm --format=just-symbols %t.a | \
+# RUN:   FileCheck --check-prefixes=ARCHIVE-BOTH -DFILE=%basename_t --match-full-lines %s
+# RUN: env OBJECT_MODE=any llvm-nm --format=just-symbols %t.a | \
+# RUN:   FileCheck --check-prefixes=ARCHIVE-BOTH -DFILE=%basename_t --match-full-lines %s
+
+# ARCHIVE-BOTH:      [[FILE]].tmp_xcoff32.o:
+# ARCHIVE-BOTH-NEXT: var_0x1DF{{[[:space:]]}}
+# ARCHIVE-BOTH-NEXT: [[FILE]].tmp_xcoff64.o:
+# ARCHIVE-BOTH-NEXT: var_0x1F7
+
+--- !XCOFF
+FileHeader:
+  MagicNumber:       [[FLAG]]
+Symbols:
+  - Name:            var_[[FLAG]]
diff --git a/llvm/test/tools/llvm-nm/option-X-Non-AIX.test b/llvm/test/tools/llvm-nm/option-X-Non-AIX.test
new file mode 100644
--- /dev/null
+++ b/llvm/test/tools/llvm-nm/option-X-Non-AIX.test
@@ -0,0 +1,67 @@
+# REQUIRES: !system-aix
+
+## Test the default "-X" option in non AIX OS.
+
+# RUN: llvm-as -o %t32.bc %p/Inputs/bitcode-sym32.ll
+# RUN: llvm-as -o %t64.bc %p/Inputs/bitcode-sym64.ll
+
+# RUN: llvm-nm --format=just-symbols %t32.bc %t64.bc | \
+# RUN:   FileCheck %s -DFILE1=%t32.bc -DFILE2=%t64.bc --check-prefixes=BITCODE32,BITCODE64
+
+# BITCODE32:      [[FILE1]]:
+# BITCODE32-NEXT: C32
+# BITCODE32-NEXT: foo32
+# BITCODE32-NEXT: undef_var32
+
+# BITCODE64:      [[FILE2]]:
+# BITCODE64-NEXT: C64
+# BITCODE64-NEXT: bar64
+# BITCODE64-NEXT: static_var64
+
+# RUN: yaml2obj --docnum=1 -DCLASS=ELFCLASS32 %s -o %t_elf32.o
+# RUN: yaml2obj --docnum=1 -DCLASS=ELFCLASS64 %s -o %t_elf64.o
+
+# RUN: llvm-nm --format=just-symbols %t_elf32.o %t_elf64.o | \
+# RUN:   FileCheck %s -DFILE32=%t_elf32.o -DFILE64=%t_elf64.o --check-prefixes=ELF32,ELF64
+
+# ELF32:      [[FILE32]]:
+# ELF32-NEXT: ELFCLASS32_var
+
+# ELF64:      [[FILE64]]:
+# ELF64-NEXT: ELFCLASS64_var
+
+--- !ELF
+FileHeader:
+  Class:   [[CLASS]]
+  Data:    ELFDATA2LSB
+  Type:    ET_REL
+Symbols:
+  - Name:    [[CLASS]]_var
+
+# RUN: yaml2obj --docnum=2 -DFLAG=0x1DF %s -o %t_xcoff32.o
+# RUN: yaml2obj --docnum=2 -DFLAG=0x1F7 %s -o %t_xcoff64.o
+# RUN: rm -f %t.a
+# RUN: llvm-ar -q -c %t.a %t_xcoff32.o %t_xcoff64.o
+
+# RUN: llvm-nm --format=just-symbols %t_xcoff32.o %t_xcoff64.o | \
+# RUN:   FileCheck --check-prefixes=BOTH %s -DFILE32=%t_xcoff32.o -DFILE64=%t_xcoff64.o --match-full-lines
+
+# BOTH:        [[FILE32]]:
+# BOTH-NEXT:   var_0x1DF
+# BOTH-EMPTY:
+# BOTH-NEXT:   [[FILE64]]:
+# BOTH-NEXT:   var_0x1F7
+
+# RUN: llvm-nm --format=just-symbols %t.a | \
+# RUN:   FileCheck --check-prefixes=ARCHIVE-BOTH -DFILE=%basename_t --match-full-lines %s
+
+# ARCHIVE-BOTH:      [[FILE]].tmp_xcoff32.o:
+# ARCHIVE-BOTH-NEXT: var_0x1DF{{[[:space:]]}}
+# ARCHIVE-BOTH-NEXT: [[FILE]].tmp_xcoff64.o:
+# ARCHIVE-BOTH-NEXT: var_0x1F7
+
+--- !XCOFF
+FileHeader:
+  MagicNumber:       [[FLAG]]
+Symbols:
+  - Name:            var_[[FLAG]]
diff --git a/llvm/test/tools/llvm-nm/option-X.test b/llvm/test/tools/llvm-nm/option-X.test
--- a/llvm/test/tools/llvm-nm/option-X.test
+++ b/llvm/test/tools/llvm-nm/option-X.test
@@ -8,8 +8,6 @@
 # RUN:   FileCheck %s -DFILE1=%t32.bc --check-prefixes=BITCODE32 --implicit-check-not={{.}}
 # RUN: llvm-nm --format=just-symbols -X64 %t32.bc %t64.bc | \
 # RUN:   FileCheck %s -DFILE2=%t64.bc --check-prefixes=BITCODE64 --implicit-check-not={{.}}
-# RUN: llvm-nm --format=just-symbols %t32.bc %t64.bc | \
-# RUN:   FileCheck %s -DFILE1=%t32.bc -DFILE2=%t64.bc --check-prefixes=BITCODE32,BITCODE64
 # RUN: llvm-nm --format=just-symbols -X32_64 %t32.bc %t64.bc | \
 # RUN:   FileCheck %s -DFILE1=%t32.bc -DFILE2=%t64.bc --check-prefixes=BITCODE32,BITCODE64
 # RUN: llvm-nm --format=just-symbols -Xany %t32.bc %t64.bc | \
@@ -32,8 +30,6 @@
 # RUN:   FileCheck %s -DFILE32=%t_elf32.o --check-prefixes=ELF32 --implicit-check-not={{.}}
 # RUN: llvm-nm --format=just-symbols -X64 %t_elf32.o %t_elf64.o | \
 # RUN:   FileCheck %s -DFILE64=%t_elf64.o --check-prefixes=ELF64 --implicit-check-not={{.}}
-# RUN: llvm-nm --format=just-symbols %t_elf32.o %t_elf64.o | \
-# RUN:   FileCheck %s -DFILE32=%t_elf32.o -DFILE64=%t_elf64.o --check-prefixes=ELF32,ELF64
 # RUN: llvm-nm --format=just-symbols -X32_64 %t_elf32.o %t_elf64.o | \
 # RUN:   FileCheck %s -DFILE32=%t_elf32.o -DFILE64=%t_elf64.o --check-prefixes=ELF32,ELF64
 # RUN: llvm-nm --format=just-symbols -Xany %t_elf32.o %t_elf64.o | \
@@ -85,8 +81,6 @@
 # BOTH-NEXT:   [[FILE64]]:
 # BOTH-NEXT:   var_0x1F7
 
-# RUN: llvm-nm --format=just-symbols %t.a | \
-# RUN:   FileCheck --check-prefixes=ARCHIVE-BOTH -DFILE=%basename_t --match-full-lines %s
 # RUN: llvm-nm --format=just-symbols -X32_64 %t.a | \
 # RUN:   FileCheck --check-prefixes=ARCHIVE-BOTH -DFILE=%basename_t --match-full-lines %s
 # RUN: llvm-nm --format=just-symbols -Xany %t.a | \
diff --git a/llvm/tools/llvm-nm/llvm-nm.cpp b/llvm/tools/llvm-nm/llvm-nm.cpp
--- a/llvm/tools/llvm-nm/llvm-nm.cpp
+++ b/llvm/tools/llvm-nm/llvm-nm.cpp
@@ -16,6 +16,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ADT/StringSwitch.h"
+#include "llvm/ADT/Triple.h"
 #include "llvm/BinaryFormat/COFF.h"
 #include "llvm/BinaryFormat/XCOFF.h"
 #include "llvm/Demangle/Demangle.h"
@@ -39,6 +40,7 @@
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Format.h"
+#include "llvm/Support/Host.h"
 #include "llvm/Support/InitLLVM.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/Program.h"
@@ -2371,17 +2373,35 @@
   UndefinedOnly = Args.hasArg(OPT_undefined_only);
   WithoutAliases = Args.hasArg(OPT_without_aliases);
 
-  StringRef Mode = Args.getLastArgValue(OPT_X, "any");
-  if (Mode == "32")
-    BitMode = BitModeTy::Bit32;
-  else if (Mode == "64")
-    BitMode = BitModeTy::Bit64;
-  else if (Mode == "32_64")
-    BitMode = BitModeTy::Bit32_64;
-  else if (Mode == "any")
+  // Get BitMode from enviornment variable "OBJECT_MODE" for AIX OS, if
+  // specified.
+  Triple HostTriple(sys::getProcessTriple());
+  if (HostTriple.isOSAIX()) {
+    auto getBitMode = [](const char *RawBitMode) {
+      return StringSwitch<BitModeTy>(RawBitMode)
+          .Case("32", BitModeTy::Bit32)
+          .Case("64", BitModeTy::Bit64)
+          .Case("32_64", BitModeTy::Bit32_64)
+          .Case("any", BitModeTy::Any)
+          .Default(BitModeTy::Bit32);
+    };
+    BitMode = getBitMode(getenv("OBJECT_MODE"));
+  } else
     BitMode = BitModeTy::Any;
-  else
-    error("-X value should be one of: 32, 64, 32_64, (default) any");
+
+  if (Arg *A = Args.getLastArg(OPT_X)) {
+    StringRef Mode = A->getValue();
+    if (Mode == "32")
+      BitMode = BitModeTy::Bit32;
+    else if (Mode == "64")
+      BitMode = BitModeTy::Bit64;
+    else if (Mode == "32_64")
+      BitMode = BitModeTy::Bit32_64;
+    else if (Mode == "any")
+      BitMode = BitModeTy::Any;
+    else
+      error("-X value should be one of: 32, 64, 32_64, (default) any");
+  }
 
   // Mach-O specific options.
   FormatMachOasHex = Args.hasArg(OPT_x);