Abstract: Penman-Monteith formula is a standard method recommended by FAO-56 to calculate potential evapotranspiration. But it is difficult to apply in practical operation as not all related meteorological elements can be obtained in reality, so estimation models are used instead. In order to apply compound drought index in operation, Thornthwaite method and Hargreaves method for yearly and monthly potential evapotranspiration of Sichuan Basin and West Sichuan Plateau are studied.The observation data of 156 meteorological stations in Sichuan Province from 1971 to 2000 are investigated including minimum temperature, maximum temperature, wind speed, relative humidity, and sunshine hours.Using result of Penman-M onteith form ula as the ET0 standard value, the precisions of the two E_T0 estimation methods are analyzed.Comparing wind speed, relative humidity, Hargreaves method and Penman-Monteith method statistically, an operational Hargreaves method is established. These Hargreaves method and model have been applied to monitor serious summer drought in 2006 in Sichuan Province.The results show: As far as annual variation is concerned, annual value fluctuation estimated by Thornthwaite method is small so that it cannot reflect the E_T0 annual variation. However, Hargreaves method and PM method show basically consistent changing tendency and can reflect the E_T0 annual variation characteristics better. As for the annual mean, result of Thornthwaite method is significantly smaller.The error in the Basin is -10 %--20 % and -40 %--60 % on Plateau.Result of Hargreaves method is relatively smaller in Plateau and larger in Basin.But its error is 10 %-15 %, better than Thornthw aite's. As for the seasonal variation, error of Thornthwaite method is larger in winter but smaller in summer.Due to the influence of relative humidity and wind speed, seasonality of Hargreaves method's error is not obvious. The error of Hargreaves is smaller than that of Thornthwaite especially in Plateau region except for in summer.Results of Penman-Monteith formula and Hargreaves method show good linear relation.With the recovery factors air velocity and relative humidity introduced, the error of Hargreaves correction value can be controlled within 10%. It can meet the demand of practical operation for precision.The compound drought index calculated by this E_T0 estimation model has strong capability for drought monitoring in Sichuan Province.