A total of 71 diatom species belonging to 18 genera were indentified from Lake Erhai in Yunnan Province (Southwest China) from February 2004 to January 2005. Results revealed a distinct seasonal succession of diatom communities, which dominated by eutrophic-tolerant species. Dominant species were different in each season:Fragilaria crotonensis was the main species in winter; Aulacoseira ambigua and F. crotonensis in spring; Cyclotella ocellata in summer; and A. ambigua and Cyclostephanos dubius in autumn. Except 1^#, the dominant diatom species were almost same in the whole lake, with a little difference in percentage between the north and the south part of the lake. It may suggest that the water quality of the whole lake was moderate to heavily polluted. Average monthly data of the diatom and water chemistry of all the sampling sites except 1^# and the RDA analysis showed that meteorological condition was the key factor, and the nutrient was the second.