Abstract:The oogenesis of Gambusia affinis is divided into 6 phases. In the first phase, the oogonium has little cytoplasm and bigger nuclear-cytoplasmic ratio. In phase 2, the volume of oocytes increases rapidly while the nuclear-cytoplasmic ratio decreases obviously, the cytoplasm is in a strong basophilic state, and the follicular cell starts to appear. To the late stage of this phase, yolk nucleus and mimetic growth circle which both would be relate to the formation of yolk matters appears. In the third phase, the basophilic of the oocytes gets weaker, and yolk bubble which the cytoplasm is gradually filled with is first seen in the inner edge of the membrane. Follicular cells which pile up as a tongue in the granular layer come to insert into egg membrane, and make the zona rediata undulate. ’This kind is called inlayed-folicle. In phase 4, the increase of oocyte volume gets sooner, the yolk bubble disappears, and cytoplasm is gradually filled with yolk and with nucleus moving to age. In phase 5,the follicular cells fall off and the eggs mature with characters of very thin membrane and well-distributed yolk. In phase 6, unfertilized eggs and follicular cells degenerate gradually and are absorbed at last.