Abstract: The species and diversity of ecological groups of planktonic Ostracoda in the waters of the southern East China Sea and the eastern waters off Taiwan Island are analyzed, the seasonal difference and regional variation are also expounded.The results show that there are four ecological groups in this sea area but the hyperthermal and hyperhaline group plays the predominant role.Meanwhile, there is a distinct difference in the abundance of planktonic Ostracoda in summer and winter, the average values are 0.63 and 1.22 ind./m³ in July and December respectively.As regarding to the horizontal distribution, its tendency exhibits that the individual density in the waters of the southern East China Sea is higher than that in the eastern waters off Taiwan Island, and a higher concentrated area is formed in the northeast part of this sea area.The community structure in this sea area was emphatically expounded.There are three ecological groups in this sea area but the off-lying community occupies the most part of the sea area.It should be specially noted that all the communities either in the vicinity of waters of Diaoyu Island or in the northern and eastern waters off Taiwan Island are the common off-lying community.In addition, the seasonal migration of the geographical position of the community habitat resulted from the changes of environmental elements in this waters is also analyzed.