Evapotranspiration plays a fundamental role in agricultural water management. Its calculation requires weather data, such as radiation, which are often not available and should be estimated indirectly. This study employed the Ref-ET software for estimating radiation for the period of 1970-2011 under two different climates of Rasht and Isfahan. Results showed that for Isfahan, the first method (minimum and maximum temperature difference) was satisfied with KRS=0.17, indicating good results. For Rasht, radiation was estimated using the third method (KRS) assuming KRS=0/44, and the evapotranspiration relative to the values of evapotranspiration in the presence of data was acceptable. Also, results of evapotranspiration derived from the Torque equation in Isfahan and results of the Penman-Monteith FAO relation for Rasht were more acceptable.