Abstract: Abstract: It is vital to explore the effect of climate change on reference crop evapotranspiration (ET0) in grasslands of different types in Inner Mongolia. According to the index of moisture raised by Ivanov, Inner Mongolia was divided into five different types of grassland including, in order of the vegetation amount, meadow steppe, typical steppe, desert steppe, steppe desert and desert. Based on the FAO Penman-Monteith equation, the distribution and temporal-spatial variation of ET0, ETrad and ETaero and the meteorological factors were studied in Inner Mongolia from 1961 to 2010. The variations of monthly and annual ET0, ETrad, ETaero and the correlated meteorological factors such as wind speed, daily average temperature, relative humidity and sunshine hours were analyzed respectively. The correlation of wind speed, daily average temperature, relative humidity and sunshine hours with ET0 were analyzed. The results showed that the mean ET0 of different sites varied from 570 mm to 1674 mm in the past 50 years, and there was a marked geographical difference in ET0 and its constituents (ETrad and ETaero), which decreased from the West to the East, crossing the desert, steppe desert, desert steppe, typical steppe and meadow steppe successively. The ET0 of those five zones during growth season accounted for 80% of the whole year and reached the peak in June while ETrad and ETaero peaked in July and May. Trends of ET0 varied from -48 mm/10a to 50 mm/10a across the Inner Mongolia. ET0 of desert, steppe desert and desert steppe decreased not significantly and those of typical steppe had an upward trend not significantly, while ET0 of meadow steppe increased significantly. ETrad increased in Inner Mongolia but ETaero decreased significantly in Inner Mongolia except meadow steppe. The ET0 of May and June decreased significantly in Inner Mongolia annually. Meanwhile, the ETrad of almost all the months in growth period increased but the ETaero decreased. The absolute values of partial correlation coefficients between ET0 and meteorological factors indicated that wind played the most important role in the changes of ET0 in the Midwest of Inner Mongolia, where ET0 of most sites experienced a downward trend with the significant decline of wind speed and the contribution of ETaero to ET0 overtook that of ETrad. As the secondary meteorological factor, the significant increase of temperature did not lead to a significant increase of ET0 in Inner Mongolia. Relative humidity, as the third factor, experienced a significant decrease and was negative to ET0. The decrease of ET0 was triggered by the reduction of sunshine hours, which was the fourth factor. Due to the combined effects of various meteorological factors, ET0 increased significantly in meadow steppe and the east of typical steppe. The present research indicated that the Midwest of Inner Mongolia including desert, steppe desert and desert steppe were facing a serious situation and the East of Inner Mongolia, a place called thriving grasslands, including typical steppe and meadow steppe were going to be exposed to the potential risk of deterioration, providing a scientific basis for the eco-environmental conservation and sustainable development of grasslands in Inner Mongolia.