Abstract: The existing methods of calculating reference crop evapotranspiration (ET₀) from meteorological data need various climatic and physical parameters. Net radiation (R_n) is one essential parameter for ET₀. However, professional measurement devices of Rn are seldom installed in agro-meteorological stations. To overcome problems in calculating ET₀ without R_n data, a genetic algorithm model was used to calibrate recommended values (a_s and b_s) of the FAO56 method, using meteorological data from Nanshaliang Meadow Steppe in the southeastern margin of Hunshandake. R_n and the corresponding ET₀ for April to September and October to March were calculated and the pre-calibration solar radiation compared with that of post-calibration. The simulation accuracy of ET₀ was analyzed using the estimated residual index method. From comparison of the result using recommended and calibrated values (a_s and b_s), a consistent annual trend was noted for simulated Rn in areas without R_n data. Although Rn obtained from calibrated values was unstable with high fluctuations, the overall accuracy of ET₀ improved. The analysis showed that large error immediately before and after precipitation events. Although precipitation factor is not directly included in the Penman-Monteith equation, it certainly influences local humidity and temperature. Apparent, both a_s and b_s were affected by changes in humidity. There was therefore the need to further analyze these parameters in relation to humidity.