A 64-years-old man had cor triatriatum (Lucas-Schmidt type I A) with severe mitral regurgitation and atrial fibrillation. We perfomed resection of the anomalous septum between the accessory chamber and left atrium, and conducted mitral annuloplasty and maze procedure. Arrhythmia were not encountered after surgery. The maze procedure and resection of the anomalous septum with mitral surgery proved to be effective for atrial fibrillation with cor triatriatum.