Abstract: The morphology and ultrastructure of the sperm ball of Cristaria Plicata was studied under light and electron microscope. The results show that the sperm ball is spherical,which is about 65—70 μm in diameter and contains up to 2300 sperms. From outer to inner, the sperm ball is composed of acellular layer, surface layer and internal region. The acellular layer is 0.6μm in width, which is brittle. The 7.5μm surface layer is partitioned into many radial chambers. Each sperm head is embedded in a chamber which points to the center of sperm ball, while the flagellum which is 37—40μm long extends away from the surface of sperm ball. The sperm plasmalemma is in contact with the surface layer in flagellar collar, no cytoplasmic bridges is observed inter connecting sperm. The internal region is spherical and about 57—62.5μm in diameter, which contains low electron-dense flocculent material. The morphological diversification of the sperm ball in fresh-water was observed under light microscope. After the sperm balls are released from outer water tube of the male C plicata, the rotational and progressive motility is achieved by sequential flagellar beat in a definite direction. When the sperm ball is barely released from outer water tube of the male C plicata, the sperm ball is a whole structure and moves quickier. After thirty minutes a handful of sperm balls became half-baked. After three hours almost 50% sperms in few of sperm balls had been released, its direction became driving-movement ahead because of disappearance of a single-side sperms and its speed became lower. After four and half hours sperms in tiny sperm balls had completely been released,they becomed a hollow-spherial structure. It was continuously observed after ninty hours, most sperm balls contained a few of sperms which moved ceaselessly, and existed also a certain number of hollow-spherial structure.On hundred and twenty hours ofter it was observed one handred and twenty hours later, sperm in almost all sperm balls had completely been released.