diff --git a/MdePkg/Include/Protocol/FirmwareVolumeBlock.h b/MdePkg/Include/Protocol/FirmwareVolumeBlock.h
index c2c1142acc..325f927adc 100644
--- a/MdePkg/Include/Protocol/FirmwareVolumeBlock.h
+++ b/MdePkg/Include/Protocol/FirmwareVolumeBlock.h
@@ -206,7 +206,7 @@ EFI_STATUS
   IN CONST  EFI_LBA                             Lba,
   IN CONST  UINTN                               Offset,
   IN OUT    UINTN                               *NumBytes,
-  OUT       UINT8                               *Buffer
+  IN OUT    UINT8                               *Buffer
 );