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This study focuses on assessing trends of reference evapotranspiration (ET o ) considering aridity. Weather data sets of 54-62 years of Inner Mongolia, a Chinese Province where climate varies from hyper-arid in the West to wet sub-humid in the East, were used. Trends were analyzed for ET o computed with the FAO Penman-Monteith method (PM-ET o ) using full data sets of maximum and minimum temperature (T max and T min ), sunshine duration (SD) used to compute net radiation, relative humidity (RH) and wind speed (WS). Trends were also assessed for ET o computed with the Hargreaves-Samani temperature eq. (ET o HS ) and the Penman-Monteith equation with temperature estimates of solar radiation and actual vapour pressure (ET o PMT ). In addition, trends relative to T max , T min , SD, RH and WS were assessed. Trends for PM-ET o show to vary with aridity, with decreasing trends in the areas marked by aridity in the West and increased trends in less arid and sub-humid areas in the East. The detected trends are well explained by the trends in weather variables which consist of large increasing trends of T max and T min and of decreasing trends for SD, RH and WS. Therefore, negative trends of ET o occur where impacts of increases in temperature and decreases in RH are smaller than impacts of declining SD and WS; otherwise, when warming influences are larger it results a positive trend for ET o . Trends were coherent when considering seasonality influences. Contrarily, results for the temperature methods, ET o PMT and ET o HS, always Water Resour Manage identified increased trends for ET o due to warming effects. These results show that it is inappropriate to assess ET o trends when using simplified temperature methods.