An experiment was conducted to investigate the effects of competitive interactions of Hydrilla Verticillata and Vallisneria natans on the phosphorus concentration both in overlying water and pore water.The aquatic plants were planted in a single-speciesmonoculture and a mixed-species pattern.Results showed that:the overlying water phosphorus concentration was much lower in the mixture treatment than either of the monospecific species treatment(p ＜0.05 ),which indicates that the competitive interactions treatment had strong,er purification ability than that of the individual plant species treatment;in contrast,total phosphorus concentration in the pore water was much higher in the mixture than that of single-species treatments.Which means that the competitive interactions of aquatic plants decreased the depletion ability of phosphorus in pore water compared with the single species.