Fourteen cases of synoviosarcoma were investigated by electron microscopy and immunohistochemical technique (PAP) using keratin and EMA as epithelial markers, vimentin as a mesenchymal marker. Of the 14 cases 9 were biphasic (BS) and 5 were monophasic synoviosarcoma (MS). In BS, epithelial-like cells, glandular, tubular of slit-like structures, junctional complex, microvilli at the luminar surface and basal lamellae at bottom could be easily seen. Spindle cells both in BS and MS were similar. These cells had somewhat similarity with epithelia-like cells, such as slender cell processes or microvilli, the external lamellae and cell junctions. The spindle cells also formed some slit-like structures. Collagenization was mild in matrix. Whether in BS or MS, epithelial or spindle cells, there were positive reactions for keratin, EMA and vimentin. The present study suggests that synoviosarcoma is neither a tumor of synovium origin nor a sarcoma of synovial differentiation, but a carcinosarcoma or adenosarcoma of soft tissues; the monophasic type is not a variant of fibrosarcoma either. It really is a distinct variant of synoviosarcoma. The expressions of both epithelial and mesenchymal features are useful criteria for diagnosis of synoviosarcoma.