Abstract: A simulated plant-water-sediment micro-ecosystem was established in an experiment using an alien species of submerged plant, Hydrilla verticillata to study the effects of the diurnal changes in physiological activities of H. verticillata on migration and transformation of nitrogen and some other environmental factors in water. Results show that the presence of H. verticillata significantly increased DO concentration, pH, and Eh, but lowered TDS. The diurnal variations of water pH, DO and Eh all displayed a single-peak curve. The peaks appeared all around 15:00. In addition, the concentrations of total nitrogen (TN) and NH₄⁺-N remained nearly constant but relatively lower than that of the control during the whole day, . Tthe mean concentration of NO₃^--N was relatively higher between 15:00 to 18:00, and the mean concentration of NO₂^--N relatively lower between 24:00-2:00 next day as compared to the control.