Changtan Reservoir is one of importance drinking water sources in eastern area of Guangdong Province and as well as a national water conservation area. The community structure characteristics of plankton in the Changtan Reservoir was determined and used to assess eutrophication status of the water resource. Water was sampled at 8 point in the reservoir In July 2010 and plankton species composition, abundance and distribution were investigated. Total of 55 species belonging to 6 phyla of phytoplankton were identified. The species belonged to Chlorophyta, Cyanophyta, Cryptophyta and Bacillariophyta were dominated, while species in Dinophyta and Euglenophyta were much less. The density of the phytoplankton was varied from 1.71×10⁶ to 2.44×10⁸ cells/L. The diversity index (H') of phytoplankton indicated that water sampled at Shaping station and Zhuyefeng station were highly polluted. At the same time, 44 species zooplankton belonging to Rotifera, Cladocera and Copepoda were identified. Among the zooplankton, Rotaria was dominant phyla. The indicators species of eutrophication, such as Brachionus forficula, B.caudatus, Filinia longiseta, Keratella valga, and Trichocerca similis were widely distributed in the Reservoir. The abundances of zooplankton were ranged from 40 to 1 588 ind/L. According to the comprehensive trophic state index (TSI) and the plankton community structure analyses, the water in the Changtan Reservoir was slightly eutrophied. The results of Canonical Correspondence Analysis on relationship between phytoplankton and environment indicated that the distribution of phytoplankton in Changtan Reservoir was strongly affected by environmental factors.