Synovial sarcoma with extensive osteoid production is rare. We report a case of synovial sarcoma of monophasic type with massive ossification. The diagnosis was confirmed by reverse-transcripitase polymerase chain reaction (RT-PCR). The patient was an-81-year-old woman with recurrent synovial sarcoma in her right knee. The tumor was primarily excised in 1989. It recurred and was removed again in 1996. However, in 1999 a painful mass appeared in the same site. Preoperative plain radiography and computed tomography revealed a 5 x 5 cm soft tissue mass with extensive ossification in the medial side of the right knee joint. The tumor was widely excised in 2000. Soft X-ray examination revealed a trabecular pattern of ossification in the excised tumor. Microscopically the tumor was composed of hypercellular spindle cells with fascicular arrangement with prominent ossification but no epithelial component. The tumor cells were positive for vimentin and focally positive for cytokeratin. The tumor expressed a sequence of SYT-SSX1 fusion gene transcript demonstrated by RT-PCR. Twelve years long survival of the present case without metastasis in spite of repeated recurrence suggests a better prognosis of synovial sarcomas with ossification.