Left atrioventricular valve aneurysm is rare. We present a rare case of partial atrioventricular septal defect with an extremely thin left atrioventricular valve aneurysm mimicking valve perforation. The preoperative echocardiography demonstrated severe left sided atrioventricular valve regurgitation on the “cleft” and leaflet perforation. But in per-operative, we discovered a left sided atrioventricular valve aneurysm instead of valve perforation. The “cleft” edge and a aneurysm were closed.