Abstract: The results of an investigation of phytoplankton flora from 2001 to 2002 in Dianchi Lake,China are reported.The results showed that 107 species and varities belonging to 6 phyla had been identified during the survey.Among them there are 50 species and varities in Chlorophyta and 33 species and varities in Bacillariophyta.But Microcystis Kutz.,Cyanophyta,was the highest in cell quantity with 84.27% of the total.The type of the community of phytoplankton in Dianchi lake was cyanobacteria-green algae.And the cells of phytoplankton were miniaturization.The number of species is higher in spring and summer and lower in autumn and winter.There were a few differences in spatial variation of phytoplankton species.