Abstract: The Three G orges Reservoir,located at 29° 16'-31° 25' N and 106° 50'-110° 50' E,is the largest water conservancyforirrigation works and generate electricity in China. It has a total area of 54061.5km2and a water area of 189km2, when the normalpool level is 175m. In order to understand the diversifications and effects of sluice on water environment and aquatic ecosystem,the authors made a preliminary studyon the phytoplanktonfeatures,including species richness,dominant species,cells and diversity index,in the Hubei river section of the Three G orges Reservoir during the period before sluice(2002. 1222003. 06)and aftersluice(2003. 0622004. 05). The results showed that the species richness,composition and cell densities of algae were changedconsiderably throughout the studied period. Before sluice,in total of 79 species of algae were observed,belong to 7 phyla and 66genera. Diatoms,green algae and blue-green algae occupied 34.6 %,38.5 %and 10.3 % of the total,respectively. Other fourphyla were only representing 16.6 %. After sluice, the species richness of algae were increased to 151 species,belong to 8 phylaand 103 genera. The percentage of diatoms,green algae and blue-green algae was 23.8 %,55.0 %and 9.9 %to the algae flora,respectively. The species richness of diatoms was decreased,but that of green algae was increased. The cell densities of algaewere the following situation:Before sluice, the monthly mean values were reached to 27216×104ind./L in the main channel,andto 1042×104ind./L in the bay region. After sluice, the values were raised to 38418×104ind./L and 200617×104ind./L,respectively,increased by 41.2 % and 92.6 %than that before sluice. The concentrations of nitrogen and phosphorus were in therange of 2.39-2.54mg/L and 0.11-0.18mg/L in the main channel,and 1.42-1.84 mg/L and 0.18-0.30mg/L in the bayregion,respectively. Those were much higher than the values in normal water bodies. The trophic status of the studied river section is at mesotrophy level in the main channel and atα 2 β-mesotrophy level in the bay region. The results illuminated that theprocess of valley changing into reservoir would be surely to affect the ecological characteristics of algae in aquatic ecosystem.Therein,the effects were more notably on bay region. Some prevention and cure measures and advices were also given in the paper,for protecting the water quality of the Three G orges Reservoir.