diff --git a/MdePkg/Include/Library/PeiServicesLib.h b/MdePkg/Include/Library/PeiServicesLib.h
index 23eeb6be00..220ac96718 100644
--- a/MdePkg/Include/Library/PeiServicesLib.h
+++ b/MdePkg/Include/Library/PeiServicesLib.h
@@ -109,7 +109,7 @@ PeiServicesNotifyPpi (
 EFI_STATUS
 EFIAPI
 PeiServicesGetBootMode (
-  OUT EFI_BOOT_MODE          *BootMode
+  IN OUT EFI_BOOT_MODE          *BootMode
   );
 
 /**