Three hemdroel 300 individuals of Oxyurella tenuicaudis of both sexes and 3 ephjppia were selected from the plankton sample collected from a small pond in Yongsheng, Yunnan province [China] on 26 Aug. 1982. Among them 265 are females occupied 88.3% of the total number of individuals. The body lengths are between 0.24 to 0.50 mm, but most of them are from 0.36 to 0.45 mm. There are 110 females with eggs or embryos in their brood pouches, which occupy 41.5%, Lengths of their body are between 0.30 to 0.49 mm. There are 64 females with only one egg occupying 58.2%, most of them are 0.36 to 0.39 mm long. 46 individuals each with two eggs are covering 41.8%, most of them are 0.41 to 0.45 mm long. There are 35 males among those 300 Oxyurella tenuicaudis, occupying 11.7%, their length are between 0.32 to 0.35 mm. Biological characters such as ratio of juvenile and adult, body length and brood size and sex ratio etc in the population of Oxyurella tenuicaudis are discussed, We infered that the population dynamics is of monocycle and begin to decline. The peak of population density has been passed whereas the males were continously increasing and the population would disappear in winter. The post-abdomen of 200 females were observed carefully and their variation were described.