This paper illustrates the diversity of planktonic Ostracoda species and ecological groups and their distribution features in China's seas and adjacent waters, including the Huanghai Sea, the Donghai Sea, the weatern Taiwan Strait, waters around Taiwan, Bashi Strait, the Nanhai Sea and the Beibu Bay. Based on the above description, a large scale ecological study is made. This paper records 183 species of Ostracoda, belonging to 52 genera, 8 subfamilies, 4 families and 2 suborders. The number of species varies greatly in different sea areas, with the largest number in the Nanhai Sea (122 species) and the least number in the south Huanghai Sea and the northeast Beibu Bay (7 species). This paper illustrates the diversity of ecological groups of planktonic Ostracoda in these waters and classifies them into 4 groups as follows: 1) Hyperthermal and hysaline group. As the most important group and largest number of species (accounting for 46.8% of the total number), this group is mainly distributed in the waters under the control or impact of the Kuroshio and is concentrated in the waters near the 200 m isopleth and its offshore oceanic waters. 2) Huposaline and warm-water group. The number of species in this group accounts for 31.2% of the total number though the number of individual in most species is very low and the individuals are mostly distributed in the nearshore warm water. 3) Eurythermal and eurysaline group. Although this group has very low number of species as it accounts for only 1.7% of the total number, the number of individual in some individual species is always very high. Some species have become the most dominant ones in many sea areas and are concentrated and migrate in the ecologically mixing zones. 4) Hypothermal and hypersaline group. This group is composed of deepwater and relatively deepwater species and accounts for 20.2% of the total number. The group has very low number of individual and is generally distributed in the waters lower than 500 m layer, with some of species only occur in the 1000～4000 m layers.