Abstract: The Three Gorges Reservoir was filled at 1 Jun., 2003. The water level of the reservoir rose up to 135m. Thelotic river ecosystem evolved into relative lentic reservoir ceosystem before the dam. The community composition of aquaticlife changed with the environmental variation. We sampled the cladocerans at 5 sites from Maoping to Guizhou everymonth during June, 2003 to July, 2004. There only 2 species were sampled before impoundment at the 5 sites in the reservoir.This result showed the lotic ecosystem didn. t suit for inhabit for cladocerans. There 20 species were sampled afterimpoundment at the 5 sites in the reservoir during the first year.The reservoir ecosystem supplied habitat for the waterflea. The cladoceran composition changed with season and distribution after impoundment in the reservoir. The species exceptBosmina coregoni and B. fatalis varied with season and showed different in composition. The species enriched at thef irst month (June, 2003) after impoundment and in the spring (April, 2004). Six species (D. brachyurum, Daphnia cucullata,Bosmina coregoni, B. fatalis, Bosminopsis deitersi and Alona guttata) distributed in five sampling sites. The othersspecies distributed in part sampling sites.