Abstract: Lake Suganhu is a closed inland ecosystem situated in the northern of Chadm Basin,the largest plateau inland lake in Gansu Province,China.It is an important habitat for transient birds and other animals.There are two lakes called Lake Suganhu,ie big and small one,which are over 110km2 totally.Most of water is originated from Haerteng River thawed from the glacier of Qilian Mountains.It will cause the decrease in the amount of water into Lake Suganhu to build a catchwater on the upriver of Haerteng River.Therefore,it is necessary to understand the community composition of phytoplankton in the lake.Phytoplankton species composition,diversity and Spatio-temporal distribution were investigated in spring(May),summer(August) and autumn(October) of 2006 in this paper.A total of 61 and 68 species(including subspecies,variety and form) were identified in the big and small Lake Suganhu,respectively.In big Lake Suganhu,there were 18 genera and 37 species in the phylum Bacillariophyta,11 genera and 15 species in the phylum Cyanophyta,7 genera and 8 species in the phylum Chlorophyta.In small lake,16 genera and 42 species belonged to the phylum Bacillariophyta,9 genera and 13 species ascribed to the phylum blue-green algae,6 genera and 11 species affiliated to the phylum Chlorophyta,and the only two species appeared in the phyla Euglenophyta and Pyrrophyta.In the two lakes,lower cell densities were found in spring and autumn,and more number was found in the season of summer during the three samplings,indicating the summer is the best season for algae growth and reproduction.The Bacillariophyta was the dominant phylum absolutely in big lake,which accounts for Ca 41.5% of the collectivity even in August when the amounts comparatively were small.The Cyanophyta was in the next place,accounted for 56.1% in the summer(August).Accordingly,there was similar trend in species composition in small Lake Suganhu,the Bacillariophyta accounted for 70% of the collectivity,and the Cyanophyta occupied 20.1% was the second in August.The index of Chlorophyta in big Lake Suganhu(4.00) was larger than that in the small(3.67).However,the index of algae species was larger in small Lake Suganhu(8.67) than that in big Lake Suganhu(5.00).These two indices suggested that the lakes were a little eutrophic.The average Shannon index H' ranged from 1.823 to 2.192 in big Lake Suganhu,and from 2.086 to 3.036 in the small,likewise,the Pielouu's index varied from 0.663-0.701 and 0.780-0.863 in big and small lake,respectively.Shannon index and Pielou's evenness index showed some difference within seasons.The Shannon's index in October was statistically significantly higher than in May in two lakes.Shannon index and Pielou's evenness index were found in small Lake Suganhu.The possible reason might be the lower saline content,and in addition,the diversity index was gradually increased from the edge to the center of the lake,it may attribute to more nutrition and minerals in the edge of the lake than the center.