Abstract: Accurate estimation of reference crop evapotranspiration(ET0) is essential to water resources planning and farm irrigation scheduling. Based on the meteorological data from 1953 to 2008, ET0 in Manas river basin (a typical inland river in Xinjiang province) was calculated by Penman-Monteith method (PM), and the meteorological factors affecting ET0 was analyzed by the path analysis. The results showed that temperature was the main factor affecting ET0. Assuming the ET0 calculated by PM as the standard value, values of monthly ET0 calculated by Hargreaves method were higher, especially from April to October per year. Based on meteorological data of 35 years, the Hargreaves method was modified by the Bayesian method, and was validated with the meteorological data of the other time series of 21 years. Results indicated that the accuracy of the modified Hargreaves method in calculating ET0 was greatly improved. The modified Hargreaves method that is simpler than PM method can be used to calculate ET0 in other inland river areas.