The most precise and simple method of evaporation measurement is application of pans in which class A pan is used in Iran. In contrast, there are many empirical methods which are used for estimating the evapotraspiration. In this study, Thornthwaite method was used due to the simplicity of its parameters and then we tried to use it for evaluating evapotranspiration by changing it's parameters. Because of its unproper estimation in Iran's climatic conditions in comparison with pan data we concluded that a considerable percentage of monthly evaporation with thornthwaite method with corrected alfa in different stations, have a good conformity with monthly evaporation measured with class A pan in the error range of 30% and in we added 0.5 unit to the exponent of Thornthwaite formula to having acceptable results for arid and semi arid region. In fact, the formula is not efficient in measurement of evapotranspiration without correction.