The incidence of a pulmonary leiomyosarcoma as a primary lung tumor is quite rare. We report a case of primary leiomyosarcoma with a cardiac invasion in a 76 year old man. He was admitted due to left anterior chest wall pain for one month. Chest computed tomography showed a 9×8×10cm sized, large round mass in the left upper and lower lobes, and an amorphous low density lesion within the left atrium. Chest magnetic resonance imaging showed a large round mass in the left upper and lower lobes with growth into the left atrium. A diagnosis of leiomyosarcoma with prominent osteoclast-like giant cells was made based on the microscopic and immunohistochemical findings of a permanent specimen by explothoracotomy. The pathologic features of the tumor showed round mononuclear hyperchromatic cells and multinucleated giant cells that resembled osteoclasts. The immunohistochemical staining showed that the giant cells are positive for CD68 but negative for the muscle markers while the round cells were positive for the muscle marker. The patient refused further treatment and died after two months.