A Japanese girl first pointed out the mass in the left atrial appendage by the echocardiography on the 3rd day of life. At 30th day of life, the mass in the left atrial appendage was excised en bloc successfully under cardiopulmonary bypass because it appeared mobile. Its pathologic finding revealed an old organised thrombus. She is doing well with no episode of thrombus formation or thromboembolism, 2 years postoperatively.