The combination of type IA cor triatriatum (Lucas-Schmidt's classification) and unroofed coronary sinus is very rare in congenital heart diseases. We have presented the first successful surgical treatment for an infant who had this combination. The patient was 10-month-old infant who admitted with cardiac murmur and short stature. Echocardiography revealed that an abnormal septum in the left atrium and dilated coronary sinus. Under cardiopulmonary bypass she underwent surgical treatment which consisted of a resection of the intra-left-atrial diaphragma, closure of ASD with an equine pericardial patch. During operation, unroofed coronary sinus (partially unroofed terminal portion of the coronary sinus) was revealed. Her postoperative course was uneventful.