A 48-year-old woman presented with abnormal electrocardiogram was diagnosed as having a left atrial tumor by echocardiography. She was asymptomatic and had no history of cardiac abnormality. Transthoracic echocardiography revealed a relatively hyperechoic and heterogenous tumor with the diameter of 5~6 cm originated from the left atrial septum but could not detect atrial septal defect. Transesophageal echocardiography showed atrial septal defect of fossa ovalis but failed to uncover shunt flow behind the tumor. We diagnosed as left atrial myxoma complicated with atrial septal defect, and an operation was performed through small right intercostal thoracotomy. The tumor was excised and the atrial septal defect was completely repaired after pulmonary vein isolation. The post-operative course was uneventful. Cardiac myxoma coexisting atrial septal defect is rare, and preoperative transesophageal echocardiography is considered essential for the diagnosis of coexistent lesions especially in the patients minimally invasive cardiac surgery is planned.was uneventful. Cardiac myxoma coexisting atrial septal defect is rare, and preoperative transesophageal echocardiography is considered essential for the diagnosis of coexistent lesions especially in the patients minimally invasive cardiac surgery is planned.