A survey on phytoplankton was carried out along the main stream of the Yangtze River and inundated and backwater areas of 26 tributaries in April,2010.Species of diatoms,green algae and cyanobacteria accounted for 93.33%-93.88% of the total phytoplankton.Dominant species in the main stream,inundated area,and backwater area were 1 species in 1 phylum,six species in 3 phyla and 6 species in 5 phyla,respectively.The occurrence rates of dominant species in the main stream,inundated area, and backwater area were 15.38%,9.23% and 25.00%,respectively.The average value of phytoplankton standing stock was the highest in backwater area,followed by inundated area and the lowest in the main stream.Compositions of phytoplankton standing stock were different among the main stream,inundated area and backwater area.The proportions of green algae and diatom were the highest in the main stream and inundated area,while the proportions of cryptophyta,cyanophyta and pyrrophyta were all the highest in backwater area.Shannon-Wiener diversity indexes and Pielou evenness indexes were the highest in inundated area,followed by the main stream,and the lowest in backwater area.Shannon-Wiener diversity indexes of backwater area of 17 tributaries and Pielou evenness of backwater area of 8 tributaries were both lower than those inundated area.Water quality assessment indicated that the proportions of slightly polluted or not polluted monitoring sections in the main stream,inundated area and backwater area were 76.92%,84.62% and 47.06%,respectively.Moderate polluted or seriously polluted were found in backwater area of 19 tributaries (73.08%) and inundated area of 7 tributaries (26.92%).The influence of the Three Gorges Dam Project on phytoplankton and water quality in backwater area is greater than that in the main stream of Yangtze River.