A catastrophic flood, which was the largest one in the Changjiang (Yangtze) River basin in 20th century, took place during July—August 1998. The densities and biomasses of three plant species (i.e., Potamogeton malaianus, Vallsineria spiralisandCarex cinerascens)in Poyang Lake, which links directly with the Changjiang River, were investigated after the flood and compared to those at several corresponding times before the flood. The results obtained from the comparative analysis showed that 1) density and biomass of each species changed significantly (p<0.01); 2) effects of the flood on the aboveground parts of each species were more serious than on the underground parts, especially of the submersed species; and 3) the flood influenced seriously the recovery of the submersed plants in the next year.