This study is aimed to identify and assess biodiversity of phytoplankton communities and the collected information is also used as a baseline for plant management and conservation of the Cu Lao Dung mangrove ecosystem in SocTrang province. Phytoplankton samples were collected twice, in the rainy and dry season, with the total of 17 sites per season in the warp, the estuary and the mangrove. 82 species belonging to 42 generas, 16 families of 3 divisions, in which 62 of diatoms, 11 of green algae and 9 of blue-green algae were determined. Diatoms was the most abundant in all sites of the two sampling seasons (71-90%). The major families found were Nitzschiaceae, Tabellariaceae, Naviculaceae, Fragillariaceae. The biodiversity of phytoplankton communities was very low in the rainy season (H? =0.199), because the diatom was dominated in this season (only 19 generas with 95%) while in the rainy season the biodervesity increased (H?= 1.092). In general, the phytoplankton density was low and highly varies in all sites taking sample (1,500-194,444 ind./L). The mean was 39,897 ± 60,200 ind./L; The density was the highest but with several changes in taking sample sites in the warp (46,283 ± 74,184 ind./L). The density was the lowest in the estuary 2,325 ± 895 ind./L. The density of diatom was dominated in all sites. There was up to 76.3% diatom in the warp, and 96.3% of them in the estuary sites. The seasonal variation of phytoplankton in mangrove ecosystem indicated that the highest numbers species and densities occurred in the dry.