Extraskeletal osteogenic sarcoma is a rare malignant tumor of soft tissue, and its predilection sites are the extremity, retroperitoneum, trunk, and the head and neck area. To our knowledge 5 cases of primary involvement of the mediastinum have been reported. Because of its rarity and difficulty in exact diagnosis preoperatively, we report an extraskeletal osteogenic sarcoma in the anterior mediastinum. The patient was a thirty eight old male. He complained of cough and sputum over 2 months. The chest roentgenogram and the chest MRI[magnetic resonance image] were done and showed anterior mediastinal mass with calcification. Excision of the mass was done under the preoperative impression of thymoma, and the pathologic report was extraskeletal osteogenic sarcoma of the mediastinum.