The study aimed to identify key water quality parameters influencing on diversity of phytoplankton in the waterbodies of Soc Trang province, a coastal region of Vietnam. The results showed that the concentrations of DO, TSS, COD, NHAt-N, NO,-N, Cl, total Fe and coliform do not meet the allowable limits of QCVN 08-MT:2015/BTNMT, Column A2. The phytoplankton composition with a total of 127 species belonging to 6 phyla of Bacillariophyta, Euglenophyta, Chlorophyta, Cyanophyta, Dinophyta and Ochrophyta, in which Bacillariophyta has largest number of species. The density of phytoplankton ranges from 42 to 92,504 individuals per litre and the species composition, the density of phytoplankton present in waterbodies in study area tends to be high in rainy season. The blue-green algae (Oscillatoria perornata) dominated and appeared at most of monitoring sites, showing high nutrient status in the waterbodies. BIO-ENVI revealed that phytoplankton species composition is mainly affected by five environmental indicators, including DO, NH,+-N, NO;-N, PO3-P and coliform. The current findings provide useful scientific information for controlling bloom of phytoplankton by limiting release of nutrients into the waterbodies.