Cortriatriatum is rare congenital heart disease characterized by the presence of a fibromuscular diaphragm that subdivides the left atrium into a proximal or "accessory" and a distal or "true" left atrial chamber. A 15 year old girl with cortriatriatum underwent surgical correction at the department of Thoracic and Cardiovascular Surgery, Soonchunhyang College in November, 1984. This case was preoperatively diagnosed as a single atrium with functional tricuspid regurgitation But on operation, we found that there were transverse septum in the left atrium through large ASD, low chamber receives the pulmonary veins, and the upper chamber gives rise to the left atrial appendage and leads to the mitral valve. And the anomalous membrane has no fenestrations. We excised completely the anomalous septum, reconstructed atrial septal defect with dacron patch and performed the tricuspid annuloplasty with DeVega method. Postoperative course was uneventful during follow up, during follow up,