The release of nutrientsand microbial enzyme activities were monitored during the decomposition of Hydrilla verticillatum incubated in tanks with sediments or sands. It can be found that the decomposition rates of sediment and sand substrates were 64. 62%and 62. 15%, respectively. And during the plant decomposition, abundant nutrients were released into overlying water, and the dissolved oxygen concentrations and oxidation-reduction potential values decreased while the concentrations of total nitrogen and total phosphorus rapidly increased. At the end of decomposition(146 d), the quality of overlying water was recovered back to the initial level. The aromatic protein was the main dissolved organic matter into overlying water at first, then the humic acid- and fulvic acid-like refractory organic compounds increased gradually. The decomposition process of plant has a relationship with microbial enzyme activities, which is related to the water quality and the characteristics of plant itself. Cellulase and sucrase have a close relationship(r=0. 70, P<0. 05), which is affected by total organic carbon and total nitrogen. Meanwhile, the catalase has a significant correlation between dissolved oxygen and electrical conductivity in overlying water(P<0. 05), while alkaline phosphatase and urease had a negatively correlation with the total phosphorus and the ammonia nitrogen respectively(P<0. 05).