Cor triatriatum is a rare congenital heart disease. A 57-years-old woman had cor triatriatum with severe mitral valve regurgitation (MR) and atrial fibrillation (AF). We perfomed mitral valve repair, left atrial appendage resection, and maze procedure by resection of the anomalous septum in the left atrium. At result, MR was controllable and AF disappeared after the operation. Although there is no established maze procedure with cor triatriatum, removing the septum was effective to complete it.