The primary mediastinal liposarcoma is very rare and only a few cases have been reported. We report a 25 year-old man who have had coughing and mild fever for a month. The simple chest X-ray and computed tomography of the chest revealed a huge mass in the right thoracic cavity that originated in the anterior mediastinum. Cytologic examination of the needle aspiration biopsy confirmed the diagnosis of a primary mediastinal liposarcoma. Through an exploratory right thoracotomy and a median sternotomy, the huge mass, lobulated and about 2,500 gm in weight, was extirpated. Mass was attached to a portion of the pericardium and diaphragm, but there was no invasion or metastasis to pleura and lung. The patient was discharged on the 12th postoperative day. He is continuously receiving radiotherapy.