A 78 year-old male was admitted to this institution for prostatectomy. He recently complained of dyspnea on effort and edema in both legs. Preoperative examination showed high blood pressure; cardiomegaly in the chest radiogram; and atrial fibrillation in his electrocardiogram. Transthoracic echocardiography showed pulmonary hypertension, slightly decreased left ventricular ejection fraction, and left atrial dilatation, but no significant mitral valve disorder. Congestive heart failure was diagnosed, and treatment with diuretics, angiotensin II receptor blocker, and digitalis was begun. Both the symptoms and cardiomegaly were improved 1 week after treatment. Transthoracic echocardiography 9 days after admission showed a 16×17 mm mobile ball-like mass in the left atrium. Transesophageal echocardiography showed a spherical mass with a stalk, which was attached to the left atrial posterior wall, near the orifice of left atrial appendage, and showed pendular motion with the heartbeat. We diagnosed the mass as a thrombus and recommended urgent surgery. The patient refused surgery, therefore, his lesion was treated with anticoagulant. He received continuous warfarin and maintained a prothrombin time (PT) international normalized ratio (INR) of 1.5 to 2.0. Transesophageal echocardiography 6 days after warfarin therapy showed that the thrombus measured only 12×14 mm. The thrombus had disappeared 4 weeks after warfarin therapy, the stalk remained and atrial fibrillation continued. The patient experienced no embolic episode or other symptoms during the follow-up period. Left atrial ball thrombus is a rare disorder associated with nonvalvular atrial fibrillation. We could verify the formation of a thrombus during short time and warfarin therapy resolved the thrombus without embolic episode.