An operated case of primary mediastinal liposarcoma is reported. A 71-year-old male who was asymptomatic was unexpectedly pointed out a huge abnormal mass shadow in the mediastinum on the chest roentgenogram, when he caught a common cold. The CT scan showed the singular horseshoe-shaped tumor in the posterior mediastinum which was adjacent to the antero-lateral phase of the vertebrae. MRI was useful to our recognition of its whole shape and localization. Extirpation of the tumor via right postero-lateral thoracotomy was performed successfully. The pathological diagnosis was liposarcoma; well differentiated, lipoma-like type. Primary mediastinal liposarcoma is a very rare mediastinal tumor (0.2%). Thirty-six cases of primary mediastinal liposarcoma including our case have been reported at present in Japan, so far as we surveyed. It is generally known that liposarcoma has the low sensitivity to the radiotherapy and chemotherapy and has high incidence of recurrence. We think that this case needs the strict postoperative follow-up.