Abstract: Heihe river basin,as one of the greatest inland rivers in China,is confronted with water resources scarceness due to the unreasonable development or utilization.To utilize the limited water resource effectively,the isotope techniques are widely used to study hydrological cycle.The variations of δ¹⁸O in different water are analyzed during the period of sampling using the data of precipitation,surface water and groundwater in Heihe river basin.It is found that δ¹⁸O is affected by rainfall, altitude,season and temperature.As is also shown through the multivariate linear regression equation,the most important factor affecting the δ¹⁸O is the temperature.The maximum,minimum and mean of δ¹⁸O in river water are lower at mountain outlets than those in mountain areas or in river basin.The change of δ¹⁸O with time in river water in mountain areas accords with that in precipitation,however at mountain outlets it is contrary to that in precipitation.The δ¹⁸O in river water increases progressively from the upper to the lower reaches of Heihe river.The δ¹⁸O fluctuates in a wide range in groundwater at Zhangye, in a more narrow range at Linze,and is divided into two parts at Gaotai.The δ¹⁸O in groundwater sampled in September is higher than those in J une at Linze.Conversely it is lower than that in June against the high δ¹⁸O part at Gaotai.Furthermore, the impact of different factors on the variations of δ¹⁸O in the process of water cycle and their interaction are evaluated in the study.Therefore,the isotopic technique can be widely applied to thorough study on the water cycle in Heihe river basin.