Abstract: From Sep.2007 to Sep.2008,structural characteristics,including species composition,cell abundance,dominate species,Shannon-Wiener index and seasonal variations,of freshwater phytoplankton were investigated in the main rivers and reservoirs of Liuxi and Bajiang River.In total,218 species belonging to 90 genera were identified.Chlorophyta species were the most common organisms,with 92 species belonging to 31 genera found,which accounted for 42.20% of total species.Cell abundance varied from 5.1 to 2090×10³ cell/L with an annual average was 152×10³ cell/L.Maximum cell abundance occurred in spring and minimum presented in summer.The Shannon-Wiener index fluctuated from 1.03 to 4.22,the Margalef index fluctuated from 1.00 to 7.83,and the Pielou index fluctuated from 0.63 to 0.85.Diversity peaked in summer and was lowest in winter.