Research results in 5 survey periods (May 2023 - February 2024) have identified 141 species of phytoplankton belonging to 60 genera, 39 families, 25 orders, 11 classes and 7 different phyla in Song Hinh hydropower reservoir, Phu Yen province. In particular, the Charophyta phylum dominates in quantity with 51 species (accounting for 36.17% of the total number of species); Chlorophyta phylum with 37 species (accounting for 26.24% of the total number of species); Euglenophyta has 19 species (accounting for 13.48%), Bacillariophyta has 15 species (accounting for 10.64%), Cyanophyta has nine species (accounting for 6.38%), Dinophyta and Ochrophyta have a relatively low number of recorded species with from 4 to 6 species, (accounting for 2.84 - 4.26%, respectively). The number of species ranges from 72 - 97 species/site. The density of phytoplankton in the Song Hinh hydropower reservoir through surveys ranged from 2,209 to 553,344 cells/litre. Seven toxic Cyanophyta species (Microcystis aeruginosa , Microcystis botrys , Microcystis wesenbergii , Oscillatoria princeps , Lyngbya sp. , Oscillatori