diff --git a/flang/lib/Optimizer/CodeGen/TypeConverter.h b/flang/lib/Optimizer/CodeGen/TypeConverter.h
--- a/flang/lib/Optimizer/CodeGen/TypeConverter.h
+++ b/flang/lib/Optimizer/CodeGen/TypeConverter.h
@@ -73,6 +73,11 @@
       // Convert to i32 because of LLVM GEP indexing restriction.
       return mlir::IntegerType::get(field.getContext(), 32);
     });
+    addConversion([&](fir::LenType field) {
+      // Get size of len paramter from the descriptor.
+      return getModel<Fortran::runtime::typeInfo::TypeParameterValue>()(
+          &getContext());
+    });
     addConversion(
         [&](fir::ComplexType cmplx) { return convertComplexType(cmplx); });
     addConversion(
diff --git a/flang/test/Fir/types-to-llvm.fir b/flang/test/Fir/types-to-llvm.fir
--- a/flang/test/Fir/types-to-llvm.fir
+++ b/flang/test/Fir/types-to-llvm.fir
@@ -293,3 +293,11 @@
 func private @foo0(%arg0: !fir.field)
 // CHECK-LABEL: foo0
 // CHECK-SAME: i32
+
+// -----
+
+// Test `!fir.len` conversion.
+
+func private @foo0(%arg0: !fir.len)
+// CHECK-LABEL: foo0
+// CHECK-SAME: i64