Cor triatriatum is a variant of abnormal connection between the pulmonary vein and true left atrium, which is separated from accessory left atrium receiving pulmonary venous blood flow by fibromuscular diaphragm. Usually it is diagnosed after operation because difficulty in visualization of the diaphragm by conventional diagnostic tools. We experienced a rare entity of congenital heart disease diagnosed as car triatriatum with atrial septal defect and partial anomalous pulmonary venous return after completing operation, which was diagnosed as ASD with PAPVR preoperatively. Anomalous right pulmonary venous opening was located at right atrium, secundum type defect of atrial septum was present, and dual chambered left atrium without connection was also seen. Excision of the diaphragm and wide patch repair of ASD including right pulmonary vein were performed with good postoperative results. Herewith, we report this case with review of literatures.