diff --git a/FatPkg/EnhancedFatDxe/Fat.h b/FatPkg/EnhancedFatDxe/Fat.h
index 6e6010dc50..ef1fd35e7e 100644
--- a/FatPkg/EnhancedFatDxe/Fat.h
+++ b/FatPkg/EnhancedFatDxe/Fat.h
@@ -425,6 +425,7 @@ Returns:
 ;
 
 EFI_STATUS
+EFIAPI
 FatOpenEx (
   IN  EFI_FILE_PROTOCOL       *FHand,
   OUT EFI_FILE_PROTOCOL       **NewHandle,
diff --git a/FatPkg/EnhancedFatDxe/Open.c b/FatPkg/EnhancedFatDxe/Open.c
index 16badf6462..0519e54981 100644
--- a/FatPkg/EnhancedFatDxe/Open.c
+++ b/FatPkg/EnhancedFatDxe/Open.c
@@ -198,6 +198,7 @@ Returns:
 }
 
 EFI_STATUS
+EFIAPI
 FatOpenEx (
   IN  EFI_FILE_PROTOCOL       *FHand,
   OUT EFI_FILE_PROTOCOL       **NewHandle,