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For calculating potential evapotranspiration (ET0), this is necessary that standard models be designed for different climate. Among the numerous methods, the Hargreaves-Samani is one that having a global validity and estimating ET0 accuracy in arid and semi-arid regions. In this study, different Hargreaves-Samani models were compared which were developed since 1982 right now. For this purpose, grass evapotranspiration from lysimeters and climatologically data for nine locations in 1988 to 1997 were used. The results of this research show that if the climatology data are measured in irrigation field, it’s better to use average minimum temperature; else it must replace this parameter with dew point temperature (T-d). In addition, initial Hargreaves-Samani (1985) method estimated potential evapotranspiration accurately when average wind speed was equal or less than 1 m per second, while it’s more than 1 m per second Modified Hargreaves-Samani by Droogers and Allen (2002) calculated better than other methods. Also, if potential evapotranspiration was being over than 6 millimeter per day, when T-d adjustments of these equations were used, the MARE and RMSD were decreased.