Abstract: In the present study, pan evaporation data and the data required for estimating reference evapotranspiration were collected from 10 meteorological stations in Khuzestan Province using the Penman-Monteith method. Three first, second and third order polynomial models were studied for conversion of pan evaporation data to grass reference evapotranspiration with pan evaporation, wind speed and air humidity as the main variables. The results obtained from the Penman-Monteith method were used as the basis for conversion. The results showed that the second and third order polynomials enjoyed higher and equal accuracy levels. Thus, the second order polynomial was recommended for use under the conditions of Khuzestan Province as it required fewer input variables. The determination coefficient for this model was found to be 0.93, its root mean square error was 0.9 mm day-1 and its correlation slope was 0.98.