Abstract: Abstract: Reference crop evapotranspiration (ET0) is of importance to estimate crop water requirement and make plans of irrigation scheme. Research on temporal and spatial variation of ET0 is important for regional agricultural water management and regional water allocation. The purpose of this study was to make a better understanding of the variation characteristics of ET0 in hilly area of central Sichuan where the agricultural water mostly depends on irrigation. Penman-Monteith equation recommended by Food and Agriculture Organization (FAO) in 1998 was applied to calculate the daily ET0 for 13 meteorological stations within 52 years (1961-2012). Kriging of GIS was introduced to draw the maps of ET0 that could present a clear spatial distribution of ET0. Mann-Kendall trend test was used to analyze temporal distribution characteristics of ET0, and the causes of ET0 change were manifested by sensitivity analysis based on path analysis method. The results showed that: annual ET0 change had a conspicuous decreasing trend in hilly area of central Sichuan in the past 52 years, especially in Langzhong,Yibin and Ziyang station where ET0 decreased significantly (α=0.05). However, in Mianyang station, ET0 increased within 52 years; The temporal distribution characteristics indicated that, during 1961－1979, ET0 increased gradually then decreased until 2003. The trend of ET0 change in a year was a single peak curve, and the change accounting for 85.82% of the year was mostly between March and October. The maximum of ET0 was 3.46 mm/d in July and the minimum was 0.79 mm/d in December. The spatial distribution of ET0 revealed that ET0 decreased gradually from northeast and southeast to the center. During 1961－1970 and 1981－1990, there were several sudden changes in spatial distribution. Compared with the average value within 52 years, the ET0 was smaller in Bazhong station but bigger in Ziyang and Neijiang station. Considering the uneven distribution of ET0 within a year and the sudden changes, the conclusion could be attained that the variation characteristics of ET0 in hilly area of central Sichuan was influenced by the change of weather greatly. The path analysis between meteorological factors and ET0 revealed that the path coefficient of sunshine duration (n), wind speed (u2), relative humidity (RH) and temperature (T) were 0.61, 0.37, -0.25 and 0.019, respectively. Their overall contribution to R2 were 0.48, 0.28, 0.13 and 0.001, respectively. When n, u2, RH and T were removed respectively in sensitivity analysis, R2 values were reduced from 0.89 to 0.596, 0.81,0.84 and 0.88, respectively. R2 had the biggest change when n was removed, and thus ET0 was more sensitive to n and n was the key meteorological factor affecting ET0 in hilly area of central Sichuan. The coefficient of determination between actual and estimated ET0 in four quarters was 0.93, 0.97, 0.96 and 0.94, respectively, when average quarter temperature instead of T was used to estimate ET0, which indicated that it was reasonable to use average quarter temperature to calculate ET0 when meteorological data in hilly areas of central Sichuan was lacking. This research provided information for agricultural production in hilly area of central Sichuan.