Abstract: Fourteen situ enclosures(9 m2 each) with different planting densities(10,20 and 30 individual plants/m2) of Myriophyllum spicatum and Ceratophyllum demersum were established to investigate the effects of plant density on control of eutrophication.The results showed that planting Myriophyllum spicatum and Ceratophyllum demersum can effectively decrease the phytoplankton biomass and improve water transparency,especially in the enclosures with 10 plants/m2 of Myriophyllum spicatum(average length 83 cm and average fresh mass 37.1 g) and 20 plants/m2 of Ceratophyllum demersum(average length 68 cm and average fresh mass 31.2 g).In these enclosures,ρ(TN) and ρ(TP) decreased significantly,water transparency and phytoplankton biodiversity increased distinctly,and eutrophication index decreased obviously.However,the submerged macrophytes with high densities could possibly cause the concentrations of chlorophyll a and nutrients to rebound.Therefore,it was concluded that rational density planting of Myriophyllum spicatum and Ceratophyllum demersum could contribute to water quality improvement in Baiyangdian Lake.