Abstract: To better understand mechanisms of plankton community response to environmental change, the plankton community structure and diversity were investigated at nine stations near Sanya city between October 26 and September 10, 2006. A total of 130 phytoplankton taxa belonging to 61 genera and 4 classes, 76 zooplankton species belonging to 9 groups, were identified in the present study. Among the phytoplankton, 101 species in 48 genera belonged to Bacillariophyta, 25 species in 10 genera belonged to Pyrrophyta, 3 species in 2 genera belonged to Cyanophyta, and 1 species belonged to Chrysophyta. The genus Chaetoceros had most abundant species, followed by Rhizosolenia. Density of phytoplankton ranged between 348–11,320 cell/L (average 3,247 cell/L) at the nine stations. Bacillariophyta dominated phytoplankton assemblages with an average of 3,230 cell/L, comprising 99.5% of the total phytoplankton abundance. As for zooplankton, species of Copepods were most abundant (29), followed by Medusae (17), Pelagic larvae (10), Chaetognaths (7), Tunicata (6), Gastropoda (4), Decapoda (1), Polychaeta (1) and Ostracoda (1). The first three were the dominant groups, contributing 28.5%, 27.7% and 13.6% of total zooplankton density, respectively. The density of zooplankton was 43–190 ind./m³ (average 114 ind./m³). The Shannon-Wiener diversity index and Pielou’s evenness index were 3.98 and 0.70 for the phytoplankton community, and 4.37 and 0.87 for the zooplankton community, respectively. Among nine stations, Luhuitou and Dadonghai had high density but low species diversity of phytoplankton. There were more species of reef-building corals and higher live coral coverage in the stations with higher plankton diversity.