Abstract: The effects of Hydrilla verticillata on water and sediment in eutrophic lake were investigated by a microcosm experiment. The experimentwas conducted with lake water, sediment and H. verticillata collected from Yuehu Lake in Wuhan, China, in 1m ?m ?m outdoor aquaria1 The apical shoots of this specieswere planted at 4 densities (0, 50, 100 and 150 shoots/m2) 1 Dissolved oxygen (DO), pH, total nitrogen (TN), ammonia (NH+4) and total phosphorus (TP) in water column were measured at the 1st, 8th, 21st, 38th and 55th day after initiation of the experiment, respectively, Eh, pH, TN and TP of sedimentwere measured aswell. At the end of the duration, the plantswere harvested and the drybiomasses were measured. The experimental results showed that the dry biomasseswere 52.9, 31.9 and 24.8 times higher at harvest than that at the initiation of the experiment respectively. DO in water column of the treatmentswith plantswas 2.6 times higher than that of the control, pH values also significantly increased up to 9.85 in the density of 150/m2 at the 38th day1 However, the 3 planting densities did not show significant difference in these two parameters1 The concentrations of TN and NH+4 in water column with plants deceased by 90.2% and 70.0% compared with the control respectively, whereas, the differences among the treatments were not significant. TP in water column decreased with planting densities by 38.9% -57.1%, and the differences among treatments were distinct. As a function of radial oxygen releases from root, the Eh values of sediment increased by 58.6-109.4mV with the planting density compared with the control.Whereas, the pH in the 3 treatments decreased sharply during the first 20 days, then increased up to 7.20-7.34, but still lower than that of the control, which could be explained by the releases of root exudates. In addition, the growth of H. verticillata can effectively reduce the TN and TP in the sediment. It is concluded that the existence of H. verticillata can remove nutrition and improve the quality ofwater column and sediment1 However, plant density is a factor need to be considered in revegetation of degraded lakes, because excessive densitymay inhibit the growth of plants1 So harvesting at appropriate time is a necessary measure to enhance the effect of the submerged macrophyte.