Abstract: It is important to understand the effect of climate change on reference crop evapotranspiration. Based on the FAO-56 Penman-Monteith equation, the temporal-spatial variation characteristics of ET0 (reference crop evapotranspiration), ETrad and ETaero and the possible causes of their changes were studied in growing season in Northwest China using a 49-year dataset. The results showed that ET0 decreased significantly in the whole study area due to the increasing trend of ETrad and the decreasing trend of ETaero. The yearly change trends of ET0 were more significant in the first region than in the other two regions because of their different climate characteristics. The absolute values of partial correlation coefficients between ET0 and four climate variables indicated that wind played the most important role in the changes of ET0 in Northwest China, followed by solar radiation and air temperature. The contributions of climate variables to ET0 in first region were similar to the whole region, while in second region the contributions of wind, solar radiation and air temperature were equivalent. The relative humidity showed larger contribution in third region than in the other two regions.