A malignant fibrous histiocytoma (MFH) is a major subset of soft tissue sarcomas, which occurs principally on the extremities or in the retroperitoneum, as well as on the head and neck of elderly patient. However, it is an extremely rare event when a MFH occurs primarily in the diaphragm of a young people. A 25-year-old woman visited our hospital complaining of right chest pain. The chest X-ray showed a diaphragmatic mass. An exploratory thoracotomic biopsy revealed a primary MFH of the diaphragm. The patient was treated with combined chemotherapy consisting of ifosfamide and doxorubicin. A partial response was seen after 6 cycles of chemotherapy. However, she died of brain metastasis 12 months after the diagnosis.