Cor triatriatum is a rare anomaly in old age. This is a case report of a 66 year-old man who had been preoperatively diagnosed as coronary artery disease and cor triatriatum. The operative findings revealed that the left atrium had an intra-atrial septum with one small opening 10mm in diameter, the upper compartment received both pulmonary veins, and there were no other anomalies like anormalous pulmonary venous connection or atrial septal defect. The patient successfully underwent open heart surgery ; the anomalous septum was resected, the mitral valve was reconstructed using French technique with Carpentier-Edwards ring, and coronary artery bypass grafting was performed.