We report a unique case of bilateral mirror image M1 aneurysms, one of which was an unruptured aneurysm arising from the proximal end of right middle cerebral artery fenestration with long loop and the other ruptured aneurysm from the contralateral side. We clipped ruptured aneurysm first and unruptured one in three months after the first operation. The difficulties of identifying this unusual vascular anomaly and possible problems during the surgery of an aneurysm at the site of fenestration are discussed with a review of the literature.