This study is dealing with the summer phytoplankton species composition in northern part of west Taihu Lake by using the monitoring data from 1991 to 1997 and the three-time monitoring chemistry data and phytoplankton data in August 1997. The results show that there are 6 main phytoplankton groups in northern part of west Taihu Lake, which are Cyanophyla，Cryptophyta,BaciUariophyla，Cklorophyia，Euglenophyta and Pyrrophyta. One reason that Microcystis spp. forming water bloom occurs every summer might be that the phytoplankton P/B ratio is higher in the Meiliang Bay. From the north part to the south part of the Meiliang Bay，the total phytoplankton biomass reduced clearly. The results also show that there is no big difference among algal species composition except that there are positive liner correlation among species in the same genus.