The evapotranspiration , as a major component of hydrological cycle, is quite important in agricultural water management and cropping pattern. The aim of this study is to asses two different climate datasets namely CRU and CFSR for estimation of potential evapotranspiration using WEAP model, in Karkheh basin southwest of Iran. Potential evapotranspiration estimated by Penman Monteith equation using observed data were considered as the evaluation criteria for WEP model outputs Potential evapotranspiration in Penman-Monteith method with observed data, was used as a criterion for evaluating the estimated values of the WEAP model. Comparisons were made using statistical indices of RMSE, R^2 and NSE between the potential evapotranspiration simulated by the WEAP model with two climate datasets CFSR and CRU and those obtained by PM equation. The corresponding values were 0.76, 0.98, 0.87, 1.08, 0.97 and 0.7 respectively, which shows that model generates better ET estimations by using the CFSR data. Comparison between study stations using RMSE, R^2 and NSE, revealed that using the CFSR dataset in Howeyzeh station with metrics of 0.44, 0.98 and 0.97, respectively and CRU data in Howeyzeh station with corresponding values of 1.19, 0.99 and 0.80, provided best model estimations. Considering the climate of station based on extended de-Martone classification, the accuracy of CRU and CFSR datasets in estimation of potential evapotranspiration were acceptable in temperate Mediterranean, moderate semi-arid and temperate arid desert climates in the study basin.