Abstract: Water samples were collected from the water area of the Dalian Lake,Shanghai in the four seasons of 2009,separately,for analysis of Pielou evenness index,Shanon-Wienner diversity index,Margalef richness index and Mcnaughton index to study community structure characteristics of the phytoplankton in the lake and of physicochemical indices to evaluate trophic state of the waterbody.Results show that a total of 64 species of phytoplankton were identified,belonging to 40 genera of 7 phyla in the water samples from the Dalian Lake.Chlorophyta was the dominant phylum,the highest in species,and followd by the phyla of Cyanophyta and Bacillariophyta.Density of the phytoplankton varied in the range from 0.35×106 to 10.87×106 L-1.Dominant species of the phytoplankton varied significantly with the seasons and their number ranged from 9 to 14.Their Mcnaughton indices,however,were not high,ranging from 0.02 to 0.60.Based on the characteristics of the phytoplankton community structure,such as density and dominant species,the water in the lake was evaluated to be between mesotrophication and eutrophication,but the evaluations based on Pielou eveness index,Shanon-Wienner diversity index and Margalef richness index differed sharply.The results of evaluation based on Pielou evenness index was basically consistent with the results of evaluation that based on phytoplankton density,species dominance and physicochemical indices,which indicates that the Pielou evenness index is suitable to evaluate eutrophication state of the waterbody of the Dalian Lake and the evaluation of Shannon-Wiener diversity index and Margalef richness index didn′t tally with the actual situation,which indicates the two indices are not applicable to evaluating eutrophication state of the waterbody of the Dalian Lake.