In order to reveal the quality and isotopic characteristics of lagoon waters in Niigata Prefecture in recent years, the nutrients concentrations of nitrogen (N) and phosphorus (P) as well as oxygen stable isotope ratios(i.e., δ¹⁸O) in water samples of Sakata and Toyanogata were measured.<br>Samples were regularly taken at the fixed sampling points from these lagoons, and depth distribution was surveyed at one point in Toyanogata. In addition, δ¹⁸O and nutrients concentrations of water samples in Zhalong Wetland at Heilongjiang Province in China were also determined.<br>Consequently, the following matters have been mainly clarified. (1) δ¹⁸O values of sample waters in Zhalong Wetland are generally large probably because of the effect of evaporation, whereas those of Sakata lagoon are large which may be due to the biological process such as the activity of plankton. (2) Considering the results of nutrients concentrations, the water quality of lagoon (Sakata and Toyanogata) waters in Niigata Prefecture is better than that of Zhalong Wetland in China.