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In this study, the performance of M5 model tree and conventional method for converting pan evaporation data (E-p) to reference evapotranspiration (ET0) were assessed in semi-arid regions. Conventional method uses pan coefficient (K-p) as a factor to convert E-p to ET0. Two common K-p equations for pans with dry fetch (Allen et al. 1998; Abdel-Wahed and Snyder in J Irrig Drain Eng 134(4):425-429, 2008) were considered for the comparison. The values of ET0 derived using these three methods were compared to those estimated using the reference FAO Penmane Monteith (FAO-PM) method under semi-arid conditions of the Khuzestan plain (Southwest Iran). The results showed that the M5 model is the best one to estimate ET0 over test sites (0.5 mm d(-1) of root mean square error (RMSE) and 0.98 of coefficient of determination (R-2). Conversely, the performance of the two K-p equations was poor.