Abstract: Reference crop evapotranspiration (ETref) is a fundamental variable in the hydrological cycle. Accurate estimation of ETref is essential to water resources project planning and farm irrigation scheduling. Based on the daily meteorological data from 123 climatic stations across Northwest China in the period of 1971-2000 and the reference crop evapotranspiration calculated by Penman-Monteith equation served as a standard, the 20 cm pan coefficient Kp was estimated by relative humidity and wind speed. And Kp model was respectively built and verified at different levels of single station, sub-region, and the whole study region. Results showed that there was a strong linear correlation between pan evaporation and reference crop evapotranspiration in Northwest China，and the correlation coefficients were 0.967. Kp model for sub-region was more accuracy than that for the whole study region, and was more suitable than that for single station. Thus Kp model for sub-region was recommend for estimating ETref in large area.