In order to evaluate the spatial and temporal variations of phytoplankton community structure in Lake Dianshan, an investigation was carried out from October 2008(autumn) to September 2009 (summer). Total of 384 species were identified. The phytoplankton community was mainly composed by Chlorophyta , Cryptophyta , Cyanophyta and Bacillariophyta . In quantity, the dominated species were Chroomonas acuta , Chlorella vulgaris all year round and Microcystis in summer. The average density of phytoplankton was 4.01×106 cells·L-1, in the order from large to small was: summer, spring, winter and autumn.The ShannonWiener index, Margalef index and Pielou index were ranged from 0.31 to 1.59，2.22 to 7.98 and 0.17 to 0.82 respectively, which meant that both diversity and uniformity of phytoplankton were in healthy status in Lake Dianshan. The structure of phytoplankton was examined by the cluster analysis. The results was that the phytoplankton community can be separated into four groups. GroupⅠ(S3，S6，S7)was in the middle and southwest area, groupⅡ(S1，S2)in the east and southeast, group Ⅲ(S4，S5) in the north and west, groupⅣ (S8) in the south. Groups were classified by quantity and dominated species of phytoplankton. Historical data was also used to analysis the longterm variation of phytoplankton community.