The phytoplankton of Lake Ulungur was investigated seasonally from November, 2006 to July, 2007. A total of 164phytoplankton species (including a number of unconfirmed species) was detected, which belonged to 90 genera, 8 phylum. Amongthem, Anabaena bergii was a new record species of cyanophyta in Xinjiang Uygur Autonomous Region, and two species ofbacillariophyta, Chaetoceros sp. and Thalassionema sp., belonged to marine algal. In each season, the phytoplankton compositionwas dominated by Chlorophyta and Bacillariophyta, secondly dominated by Cyanophyta, Euglenophyta, Xanthophyta, Chrysophyta,Cryptophyta and Pyrrophyta. The number of phytoplankton species was the highest in autumn (141 species), followed by the summer(128 species) and spring (121 species), the lowest in winter (95 species). There was an obvious seasonal characteristiscs for thedominant species. Raphidiopsis sinensia, Teraëdron minimum, Selenastrum minutum, Chlorella vulgaris and Synedra acus were dominant species in the whole year, while Synedra ulnacould only be found in spring, summer and autumn. Kirchneriella contorta and Chlorellaellipsoidea dominated in summer, and Chlorococcum infusionum and Treubaria crassispina dominated in winter. The standing cropof phytoplankton peaked only in summer with a biomass of 6.77mg/L and the minimum biomass of 1.45mg/L occurred in winter.Compared with historical data, the phytoplankton composition had remarkable changes. The growth of green alga and diatom weremost notable, while the number of the phytoplankton increased many fold in the past 30 years.