diff --git a/MdePkg/Library/HiiLib/HiiString.c b/MdePkg/Library/HiiLib/HiiString.c
index fbeb2dbdb6..2f44fabfd3 100644
--- a/MdePkg/Library/HiiLib/HiiString.c
+++ b/MdePkg/Library/HiiLib/HiiString.c
@@ -495,8 +495,8 @@ zulzu\
 EFI_STATUS
 EFIAPI
 ConvertRfc3066LanguageToIso639Language (
-  CHAR8   *LanguageRfc3066,
-  CHAR8   *LanguageIso639
+  IN  CHAR8   *LanguageRfc3066,
+  OUT CHAR8   *LanguageIso639
   )
 {
   UINTN  Index;