Based on microcosm experiments, a model for self-purification process in eutrophicated water is developed. The characteristics and applicability of the model are summarized as follows :<br>(1) The self-decomposition process in eutrophicated water is expressed by a model with 7 variables of bacteria, phytoplankton, zooplankton and 4 types of nonbiotic organic matter.<br>(2) The behaviors of nitrogen and phosphorus are also simulated by this model under the assumption of constant N and P contents in each variable.<br>(3) The model is verified by the good coincidence of simulated values with observed values with high correlation coefficient values more than 0.9.<br>(4) The role of each organism group on self-purification process is evaluated with its respiration rate calculated by this model. The contribution proportions of bacteria and phytoplankton are estimated 40-70% and 10-40%, respectively, while that of zooplankton varies widely from 0 to 40% according to sample properties.