Abstract: To investigate succession of phytoplankton in summer in Xiangxi Bay (XXB) of the Three-Gorge Reservoir (TGR) as affected by environmental factors,continuous field observations were conducted throughout the summer of 2010.Results show that biomass and Chl-a concentration of the phytoplankton in the main stream of the Yangtze River was much lower than that respectively in XXB during the algae bloom season in summer,which was mainly attributed to the fact that the water body of the main stream of the Yangtze River was well mixed,while that of XXB was weakly mixed vertically.As a result the phytoplankton in the latter grew flourishingly and evolved from diatom (in June) to green algae (in July) and then to cyanobacteria (in August).Gray correlation analysis indicates that density of the phytoplankton was closely related to thickness of the euphotic layre and the mixed layer of the waterbody with correlation coefficient being over 0.6 and that succession of the phytoplankton communities was mainly affected by water temperature,waterbody stability and thickness of the mixed layer.