A 67-year-old woman suffering from chronic atrial fibrillation and cadiomyopathy was transferred to our hospital for treatment of a left atrial thrombus. Echocardiography demonstrated the thrombus attached to the left atrial appendage, left ventricular dilatation and low ejectior fraction. Removal of the thrombus and radiofrequency Maze ablation were performed simultaneously under beating heart cardiopulmonary bypass. The patient showed no impairment of lefl ventricular function perioperatively and regained normal sinus rhythm.