In order to have a thorough understanding of the variation characteristics of reference crop evapotranspiration (ET_0) in Inner Mongolia,the Penman - Monteith method recommended by the United Nations Food and Agriculture Organization in 1998,was used to estimate the monthly ET_0 at 50 meteorological stations in Inner Mongolia during 19812012. Also,the global drought index (UNEP) proposed by the United Nations Convention on Combating Desertification was applied to carry out the climate partitions,and the spatiotemporal variation of ET_0 in different climatic zones was analyzed by using spatial interpolation and sliding average method in eight years. The results show that ET_0 variation ranges are 1 401 - 1 573,1 145 - 1 269,900 - 1 013,710 - 857 and 571 - 735 mm in special arid,arid,semi-arid,arid and humid,and moist sub-humid climate zones,respectively,with a less than 200 mm fluctuation amplitude. Additionally,the ET_0 value is gradually reduced,i. e. the more humid the climate zone,the smaller the cumulative annual ET_0. Based on the dates when the highest and lowest ET_0 have occurred,ET_0 shows strong randomness.