This study aims to analyze and compare the community structure of planktonic ostracods assemblages between Nansha reef area and open water area,and the distribution pattern influenced by ocean current movement was discussed.During April and September in 2013,27 sampling stations were set up in Nansha Islands Waters,and planktonic ostracods were collected at depths from 200 m to the surface using medium plankton net.Samples were treated and counted according to the standard methods.The results are the following findings:(1)A total,of 45 planktonic ostracods species were identified,belonging to 19 genera,4 subfamilies,and 2 families.Twenty-one species only occurred in spring and 38 species only in autumn,and 14 species occurred in both seasons.(2)The dominant species were different between two seasons;Euconchoecia maimai,Porroecia porrecta,and Conchoecetta acuminata dominated in spring,while Cypridina nami,Euconchoecia bifurata,Porroecia spinirostris,Microconchoecia curta,Metaconchoecia macromma dominated in autumn.All dominant species are either eurythemal-euryhaline or warm-hyperhaline.Studies show that,(1)There are remarkable differences in the density of planktonic ostracods between two seasons.In spring,density ranged from 0.03 to 8.25 ind/m³,averaging 1.4 ind/m³;in autumn,density ranged from 0.05 to 18.92 ind/m³ with an average of 4.11 ind/m³.High density occurred in certain limited areas due to ocean circulation.In spring,three high density areas were located in the west,northwest and southeast corners of Nansha Islands waters,respectively.In autumn high density area occupied the vast east part.(2)Species richness,diversity and evenness were 0.59,1.37 and 0.55,respectively,with much higher values in spring than in autumn.(3)Planktonic ostracods can be divided into two communities in both seasons characterizing,reefs and open waters,and seasonal variation in community distribution was impacted strongly by ocean current change.(4)The tropical cyclonic and anticyclonic circulation driven by monsoon can be regarded as the dominant factor to influence the structure of planktonic ostracods community.