To understand the ultrastructure of Schistosoma japonicum sperm.Testis and oviduct were cut from male and female of S. japonicum respectively, location was assured by observing with microscope. Specimens preparation for transmission electron microscope were made by traditional way.The sperm of S. japonicum includes two parts: a head and a single tail. The elongated ovoid head measures by 6.2 x 1.4 microns, lacking an acrosome, rounding anteriorly and tapering posteriorly. A prominent anterior mass of mitochondria lies beneath the plasma membrane. A layer of microtubes under the plasma membrane are parallel with long axis of the head. A coil of longitudinal microtubes were present beneath the plasma membrane. The nucleus is largely electron-opaque with some electron-lucent patches or less electrondense material. The tail is a single flagellum with a unique axoneme, which originated from a centriole. The axoneme in main part of the flagellum (including middle and posterior parts of the tail) consists of nine doublets and a single diffuse central structure, forming 9 x 2 + > type. But the axoneme in the transition area (anterior part of the tail) lacks central material, forming 9 x 2 + 0 type.The sperm ultrastructure in the schistosomes is homogeneous, but differs distinctly from those trematodes of other Digenea.