diff --git a/hpvm/tools/hpvm-clang/main.py.in b/hpvm/tools/hpvm-clang/main.py.in
index b20af0b80f192fe3c87b004c05a72b034aee098d..6d3c4595ead0c30ef5cca090d92cd78432ca9240 100644
--- a/hpvm/tools/hpvm-clang/main.py.in
+++ b/hpvm/tools/hpvm-clang/main.py.in
@@ -229,8 +229,8 @@ See option -b for that."""
         help="[clang emit-llvm] clang++ flags (such as -ffastmath)"
     )
     parser.add_argument(
-        "-O", type=str, default="0", metavar="level",
-        help="[clang emit-llvm] Optimization level"
+        "-O", type=str, default="1", metavar="level",
+        help="[clang emit-llvm] Optimization level. Note that default is -O1."
     )
     parser.add_argument(
         "--std", type=str,