Stable isotope ratios of oxygen and hydrogen were investigated in southern Yellow Sea in August 1997. Salinity showed good positive correlation with ${delta}^{18}$O and ${delta}$. The correlation between ${delta}^{18}$O and ${delta}$D is good. From the relationship between these parameters, we obtained two lines of conclusion: 1) seawater of study area I in summer is a mixture of Changjiang Water and modified Kuroshio Water; 2) stable isotopes are very useful tracers in studying property and behavior of water masses in the study area. In case when water masses can not be easily distinguished by T-S analysis, the stable isotopes seem to be powerful tools for this purpose.