Primary cardiac tumors are rare and about 20~25% of primary cardiac tumors are malignant. Moreover, primary malignant mesenchymoma of the heart is extremely rare. Recently, we have experienced two cases of cardiac malignant mesenchymoma. In the first case, malignant mesenchymoma which was originated from the posterior wall of the left atrium obstructing the mitral orifice was revealed pathologically in a 61-year-old woman with mitral regurgitation. The mass, which was 2.7$ imes$3.7cm in size on the posterior wall of left atrium, was extended to the posteromedial commissure and annulus of the mitral valve. The mass was resected partially without excision of the left atrial free wall. She was discharged after 30 days without any problems and she received chemotherapy and followed up for 19months. The second case was a 4$ imes$5cm in size, friable, yellow-whitish multilobulated mass in the left atrium which was originated from the left lower pulmonary vein. Multiple minor tumor nodules were found in the wall of the left atrium and the posterior leaflet of mitral valve. Partial mass excision and mitral valve replacement were performed.