The stable isotope hydrological processes in Naqu River basin,middle of the Tibetan Plateau, was studied based on the observations of δ18
O in both river water and precipitation during the summer in 1998. The fluctuation amplitude of δ18
O is much smaller in river water than in precipitation. The variation of δ18
O in river water is a combined effect of δ18
O in river water,precipitation amount and surface evaporation. The study also found the obvious impact of lake water on stable isotope homogeneity in river water. The difference between the weighted δ18
O in river water and lake water reflects the strong soil surface and lake surface evaporation in the research basin.