Abstract: To research spatio-temporal pattern of phytoplankton in the north basin of Lake Dianchi and to find out the key factor that influence the pattern, investigations were conducted 24 times at 6 sampling sites from December 2008 to June 2009. The composition and densities of phytoplankton were measured and canonical correspondence analysis (CCA) was applied to explore the relationship between phytoplankton species and the environmental parameters. By comparing annual monitoring data in Dianchi Lake, chlorophyll a concentration in the north basin started to decline at mid-December, reached the minimum (26.5 μg/L) in mid-February and the maximum (530.5 μg/L) in mid-May. A total of 97 phytoplankton taxon, belonging to 74 genera were identified, consisting of 53 species of green algae, 20 species of blue-green algae, 17 species of diatoms, 2 species of cryptomonad, 2 species of both golden algae and euglenoids, and only 1 species of dinoflagellates. Cyanobacterial biomass was dominant in December 2008 and from March 2009 to June 2009. Dominant species of Cyanobacteria were Microcystis viridis in December 2008, Aphanizomenon flos-aquae from March to May 2009, and Microcystis wesenbergii in June 2009. Diatoms peaked in January and dominant species were Melosira granulata var. angustissima. The results showed that the biomass of Microcystis increased rapidly when the water temperature was higher than 14℃; the biomass of Aphanizomenon increased with temperature while decline rapidly when water temperature was higher than 22℃. Phytoplankton distribution at the 6 sampling sites in the north basin of the Dianchi Lake was significantly different. Biomass of Microcystis and Aphanizomenon at D1 and D4 were significantly higher than that of other four sampling sites (P0.01), while biomass of D2 and D5 were significantly higher than that of the other four sampling sites (P0.01). The results indicated that, in the severe eutrophic north basin of Lake Dianchi, water temperature may be the key factor affecting the phytoplankton composition and biomass; the meteorological factors, together with uneven nutrients concentration, were considered to be the main driving forces determining the horizontal distribution of phytoplankton.