In order to investigate the characteristics of lagoon waters in Niigata Prefecture in recent years, oxygen stable isotope ratios(i.e., δ18Ο), the concentrations of dissolved organic carbon(DOC), and major ions in water samples of Sakata and Toyanogata were measured. Samples were generally taken monthly from 2004 to 2006 at the fixed sampling points from these lagoons. In addition, to survey the influence of rainfall event on these lagoons, δ18Ο and major ions in some samples were determined before and after rain in June -July, 2005.Consequently, the following matters have been mainly clarified. (1)δ18Ο value of sample waters in Sakata lagoon was larger than the values of other environmental waters in Niigata Prefecture, and was especially large in summer. It can be closely related to the biological process such as the activity of Crustacea plankton and to the recharge from surrounding groundwater as well as the effect of evaporation. (2)No remarkable hourly variation of δ18Ο was found in sample waters of Sakata. (3)The effect of rainfall event on δ18Ο(and the concentrations of major ions) in lagoons was obviously different between Sakata and Toyanogata. (4)The relationship between δ18Ο and DOC was relatively good in water samples in Sakata(as compared with that in Toyanogata).