Abstract: This study proposed a fuzzy sets model for computing ETo using only a few basic hydrological parameters including temperature, humidity and wind speed. Genetic algorithm technique was applied to calibrate membership function condition of fuzzy sets model. The proposed model was applied to determine the ETo of 5 Meteorological Stations in Thailand. The daily data of the stations were used in the study. Results showed that the fuzzy-GAs model can be used to calculate the ETo, given only the basic hydrological parameters; temperature, humidity and wind speed. The obtained ETo of the proposed model closed to the ETo of the accepted Penman-Monteith equation. Furthermore, the results presented that the Genetic algorithm calibration provided the optimal condition of membership function.