Phytoplankton are a very important component of the coastal marine food chain. Thermal water discharged from power stations has become a serious problem for the coastal environment, but we still know very little about the potential impact of thermal effluents on the marine ecosystem. In this paper, species composition, cell density of phytoplankton and chlorophyll concentration were measured during a series of temperature controlled laboratory experimental incubations in summer (July) and winter (January). The natural sea water for the experiments was collected from Xiangshan Bay (Lat. 29°38′N Long. 121°41′E) near Ningbo City, Zhejiang Province. Phytoplankton samples were preserved with Lugol′s iodine (1%) and analysed under the microscope and the chlorophyll concentration was measured according to routine protocol. The results showed that the main phytoplankton group was diatoms, with Skeletonema costatum being the dominant species. Diatom species composition was markedly influenced by temperature, which related to the seasons, the range of temperature elevation and experimental time. The species numbers declined rapidly when temperature surpassed 36℃ in summer when the ambient seawater temperature was 28℃, or 34℃ in winter when the seawater temperature was 12℃. Cell density of phytoplankton was also markedly influenced by temperature. In the summer experiment, the maximum cell density (40.56 cells/dm3) appeared at 30℃, and in winter, the maximum cell density (625.8 cells/dm3) appeared at 24℃, both of the maximums were observed in the 7th day of the experiments. The chlorophyll concentration of sea water was influenced by the ambient temperature as well, with the maximum chlorophyll concentration (42.61 μg/dm3) appearing at 24℃ in winter, which was positively correlated with the phytoplankton cell density in the winter experiment (r=0.81，p＜0.01).