Abstract: This study analyzed the species composition, dominant species, abundance, and community diversity of phytoplankton, based on the data collected from the ecological investigation of coastal waters in Northwest Hainan Island in both wet season (September, 2012) and dry season (January, 2013).The results showed that a total of 151 species among 42 genera belonging to 4 phyla were identified in the surveyed areas.Diatom was the predominant group of all the phyla.There were 90 species common between wet and dry seasons, and the species similarity index between two seasons was 59.60%, suggesting no significant seasonal variations in the species composition.During the wet season, there were 3 dominant species, Chaetoceros curvisetus (C.curvisetus), C.lorenzianus, and Nitzschia delicatissima, while C.curvisetus was with the highest dominance index (0.464).Phaeoecystis globosa and Thalassiosira subtilis were the dominant species in dry season, with dominance indices of 0.217 and 0.062 respectively.The average cells densities of phytoplankton in dry and wet seasons were 3.250104/L and 0.133104/L respectively.The abundance of phytoplankton cells showed significant regional and seasonal differences, and the main concentrated areas were Haikou and Lingao coastal waters.However, compared with dry season, in wet season, the phytoplankton density was relatively low in Haikou and Danzhou coastal waters but higher in Dongfang coastal area.In general, the Shannon-Wiener index (H') and the Pielou's evenness index (J) were 3.378 and 0.679 respectively during wet season and 2.795 and 0.578 respectively in dry season, suggesting a high diversity of the phytoplankton community.The phytoplankton diversity showed a higher seasonal variation in Lingao coastal area compared with other areas during dry season, but the opposite was observed in wet season.Correlation analysis demonstrated that the density of phytoplankton community had definite correlations to environment factors.During wet season, phytoplankton density significantly positively correlated with the dissolved inorganic nitrogen (P0.01) and chlorophyll a (P0.05); however, during dry season, it significantly positively correlated with biochemical oxygen demand (P0.01) and dissolved oxygen (P0.05), but significantly negatively correlated with salinity (P0.01).