This study was researched on the trends of water quality in the creek grown aquatic plants. The measured items were T-N, T-P, water level, temperature and rainfall. In the study site, there were many water hyacinths growing on the creek. Nutrient rich salts absorption of water hyacinths was verified by the observation of the creek water quality. The capacity of water nitrogen absorption of the water hyacinths in this site was fixed quantity by measuring nitrogen content of the water hyacinth with Kjeldahl method. To evaluate the capacity of nitrogen observation, the numerical model of removal nitrogen by the plants was estimated using the logistic curve. The comparison of the logistic curve, the effect of removal water nitrogen caused to water hyacinth was better than the paddy rice. It was effective to use water hyacinths as the system of purifying creek water quality.