Abstract: The composition and diversity of phytoplankton community were analysed in four sample stations in Donghu Lake, Wuhan from Jan. 1994 to Dec. 1996. 260 taxa belonging to 99 genera and 7 phyla were observed, of which 2 taxa were recorded for the first time in China. The composition was similar in the four stations in successive years, and the community structure of the phytoplankton in 1995 were analyzed. The number of the total species was the highest in summer, minimum in winter and the species occurred at the four stations counted for 39.6% of the total species. The species number of Chlorophyta was the highest, with high species number of Chlorococcals found (counted for 80% of Chlorophyta), but none species was considered to be dominant species. The second highest species number was found to be Bacillariophyta, of which pennate diatoms comprised more than 80% of the diversity, with less than 20% centric diatoms found. However, two centric diatoms (Cyclotella meneghiniana and Stephanodiscus hantzschii)were detected to be dominant species. Though the species number of Cyanophyta was not high, the community was dominated by Cyanophyta in Summer and Autumn in four stations, dominant species included Merismopedia minima, Phormidium tenue and Oscillatoria limnetica. Cryptophyta could be dominant groups in winter or spring though only 3 species were identified. The dominant species in stations Ⅰ&Ⅱ are different from those at stations Ⅲ&Ⅳ in Winter and Spring:dominant speices in stations ⅠⅡ are indicators of eutrophication, but some dominant species in stations ⅢⅣ were β-mesotrophic indicators, e.g. Achnanthes minutissima. Three kinds of diversity index(Margalef, Simpson and Shannon-Weaner) were calculated and results suggested that diversity index might be not good indicators of water quality when the water was at high eutrophication status. However, the Diatom Quotient declined in the order of stations Ⅰ to Ⅳ and was proposed to be highly correlated to water quality. Finally, the relationship between changes of the phytoplankton community structure and the water quality was discussed. It is concluded that the eutrophication had been aggravated since 1950s.