The phytoplankton groups were investigated in 30 shallow Chinese lakes in the middle and lower reaches of the Yangtze River area in the summer of 2003 and 2004. To explore the effects of different nitrogen phosphorus ratios (TN/TP ratios) on the phytoplankton taxonomic composition of these study sites, six main taxonomic groups were studied with three TN/TP ratios intervals: TN/TP＞30, 12＜TN/TP＜30 and TN/TP＜12. The biomass curves of these taxonomic groups showed corresponding increases or decreases with different TN/TP ratios. When TN/TP ratios declined from high (＞30) to medium (12-30), the slopes of the total biomass curve increased, as did the relative abundances of all groups except Cyanophyta. But when the TN/TP declined from medium (12-30) to low (＜12) levels, the slopes of most groups decreased except Chlorophyta and Cryptophyta. The amount of Cyanophyta increased with TP when TN/TP ratios declined from above 30 to below 12, suggesting that cyanobacteria adapted to higher TN/TP ratios. However, Chlorophyta and Cryptophyta tended to be restricted by phosphorus when TN/TP ratios declined from above 30 to below 12, and these groups adapted to lower TN/TP ratios. The biomass of Bacillariophyta, Pyrrophyta and Euglenophyta tended decreased when TN/TP ratios were above 30 and below 12, suggesting that medium TN/TP ratios (12-30) favoured these groups.