Gaozhou Reservoir is an important drinking water source of Maoming city of Guangdong province.Cyanobacteria blooms, occurring in spring of 2009 and 2010,had brought safe hidden trouble to residents.Based on the data collected from the reservoir in two springs,the highest densities of phytoplankton in the spring of 2009 and 2010 were 4.08×10⁷ cells/L and 1.47×10⁸ cells/L,and cyanobacteria densities accounted for 98.2% and 98.7% of total phytoplankton,respectively.The dominant species were Anabaena spp.and Microcystis aeruginosa in the two springs in the reservoir.The TN/TP ratios ranged from 28 to 64,phosphorus was the limiting factor for phytoplankton development.The chlorophyll-a concentration ranged from 2.5 to 31.6 mg/m³ , and the cor-relation between pH(r=0.753, P<0.01)and DO(r=0.565, P<0.05) was excellent.It can be inferred that the main reasons of spring phytoplankton blooms were the high stability of water, water temperature rising and the nitrogen and phosphorus level close to the threshold value of spring phytoplankton blooms.