According to the investigations in Kaitan reservoir area of Oujiang river, Zhejiang Province from July 2001 to June 2002, 82 species of phytoplankton were identified, which belongs to 7 division and 40 genera. The dominant species were Bacillariophyceae and Chlorophyceae. The type of phytoplankton community is Chlorophyceaetype and Bacillariophyceaetype. The speaies and numbers of phytoplankton in different seasonals and water areas showed marked variation. The nutritive peculiarity belongs to responding type of phytoplankton. The development tendency of mesotrophic condition for water body were clear with part of water body suffering in some degree from pollution.