We sampled the Yangtze River Estuary during the dry period(February-March, 1999 and February-March 2000, respectively) and the flood period (August, 1999) in order to investigate the quantitative distribution and seasonal variations of Copepods. In this study, we found that in the dry period of 1999 the average number of Copepods in the Yangtze River Estuary was 76 ind/m3. This species accounted for 95.61% of all plankton population at the time period; during the flood period of 1999, the average number was 254 ind/m3 , accounting for 84.29% of all plankton population; during the dry period of 2000, the average number was 97 ind/m3 accounting for 84.46% of all plankton population in the Yangtze River Estuary. According to the index of dominance, the dominant species was Sinocalanus sinensis in the dry period of 1999 and 2000 and Schmackeria poplesia in the flood period of 1999 ; whereas Tortanus vermiculus and Labidocera euchaeta were the dominant species in both dry and flood periods. There were 14 Copepods species found in dry period of both 1999 and 2000, and as many as 25 Copepods species found in the flood period of 1999. As the main copepods species in the Yangtze River Estuary, Sinocalanus sinensis abundance varies remarkably with seasonal variation. This study shows that lower salinity suits Sinocalanus sinensis growth. Tortanus vermiculus has rather considerable annual variation in its abundance and can grow in a much wider salinity range than that for Sinocalanus sinensis. The suitable salinity range for Labidocera euchaeta growth is similar to or slightly wider than that for Tortanus vermiculus. Labidocera euchaeta′s productivity is low in the dry period, but it becomes much high in the flood period. Furthermore, it can grow well in relatively high temperature. Schmackeria poplesia has a wide salinity range suitable for growth and can adapt to significant salinity variation. Its population is low in the dry period, but the population becomes high in the flood period. It can also grow dramatically in relatively high temperature.