Abstract: Dianchi Lake is a typical plateau, unstratificated, eutrophic lake in southwest of China.It has been seriously polluted inthe past decades.And it had not been investigated in the whole lake for phytoplankton since 1980s.Thus, a project was carriedout to find out what happened to the phytoplankton biomass and its regime dominance in the lake now.In this study, Chlorophyll a (Chl.a) had been monthly investigated according to the national standard method at 40 sites inDianchi Lake from September, 2001 to August, 2002.Cell density and phytoplankton species composition had been investigatedbimonthly as well.The results showed that Chl1a increased from 01015mg/L to 0110mg/L (monthly average values) during Jan-uary to August, 20021 Correspondingly, water temperature increased from 10 e to 28e.Chlorophyll concentrations of the southsites were lower than the north ones.Site one had the highest chlorophyll a value in all sites.We concluded that the southwesterprevailing over the Dianchi Lake played avery important role.Thehorizontal movement of phytoplankton should bewind-induced.The water blooms mainly composed by cyanobacterial species.The percentage sometimes reaches to 100%.And Microcystisaeruginosa was the most common species.But in the spring of 2002, the dominance species was changed.Aphanizomenon spp.be-came dominant (69%) in March.It showed an obvious evidence for the seasonal succession.Moreover, the investigation indicatedthat the Pearson Correlation were high significant between chlorophyll a and water temperature (01736, p<0101), total nitrogen(01970, p<0101) and chemical oxygen demand (01929, p<0101).Therefore,we concluded that phosphorus was no longer thelimited nutrition factor in Dianchi Lake to phytoplankton growth, nitrogen has taken the place.CODMn,which is considered the to-ken of organism population level, become the main chemical factor affecting the algal biomass.All the results indicatethat DianchiLake is extremely eutrophication now; the blooms can last for 9) 10 months in the year; the water is polluted seriously, especiallythe organic pollution, and it became an important factor which could affect the phytoplankton biomass directly; the species aresimplex, Microcystis, Anabaena and Aphanizomenon were the most popular bloom-forming cyanobacteria.