The community composition of phytoplankton was investigated in Danjiangkou Reservoir in December 1992, January, April and September 1993.A total of 60 genera were collected including 19 genera of Bacillariophycea, 26 genera of Chlorophycea and 8 genera of Cyanophycea.There were 42.18×10⁴ ind./L in annual mean density and 0.71 mg/L in biomass.It is showed that the number of species (37 genus) and standing crop (54.5×10⁴ind./L) in Hanjiang disrrict were higher than those (26 genera and 26.5×10⁴ ind./L) in Danjiang district.This paper also summarizes the data of phytoplankton taken from the surveys made in 1958,1986~1987 and 1992~1993 in Danjiangkou Reservoir, and discusses the main ecological changes of phytoplankton asscxriation (abundance, biomassr species composition and seasonal variation) before and after the construction of the reservoir.After dammed, the adua-dance of planktonic algae was increased by several times, from ten thousand to millions of individuals per liter.The dominance of Bacillariophycea was gradually replaced by Bacillariophycea, Cryptomonads and blue-green algae.