Primary cardiac tumor has very low incidence, especially in cases of malignancy. A 29 year old male patient visited our cardiologic clinic for recent aggrevation of dyspnea on exertion and palpitation. Echocardiography showed a large tumor in the left atrium, which suggested the left atrial myxoma. Urgent open heart surgery was taken. The operative finding was fossa ovalis based a large tumor (35$ imes$90$ imes$50 mm) that invaded the posterior wall of LA and right superior pulmonary vein directly. The tumor was excised well by simple dissection, and the final pathologic report was malignant myxofibrosarcoma. His postoperative course was smooth and he was discharged in good health. Postoperative radiation and chemotherapy had taken with satisfactory clinical outcome.