Cor triatriatum is rare congenital heart disease which is another variant of anomalous pulmonary venous return. It has abnormal fibromuscular diaphragm between true left atrium and accessary chamber which has one or more orifice to the left atrium. In classic form, the patient dies within several months after birth due to pulmonary hypertension inevitably, so it is rarely found in adult. With priopertive echocardiogram and cineangiogram we had two impressions, left atrial cystor cortriatriatum. At operative finding, there was no visible combined anomaly except accessary chamber which received all pulmonary venous return that drained into the left atrium through small calcified orifice. The operation was performed by simple resection of the diaphragm under cardiopulmonary bypass. The postope rative course was uneventful.