diff --git a/tools/x264_perf_O3.py b/tools/x264_perf_O3.py
index d637c7d..72a7b55 100644
--- a/tools/x264_perf_O3.py
+++ b/tools/x264_perf_O3.py
@@ -61,7 +61,7 @@ def main(
                     str(x264_exe),
                     *x264.specinvoke_args["ref"],
                 ],
-                output=str(Path.cwd() / f"x264-{x264_run_id}perf"),
+                output=str(Path.cwd() / f"x264-{x264_run_id}.perf"),
             ),
         ],
         check=True,