Irrigation efficiency is necessary information in water resource management. We proposed fuzzy sets approach for estimating varied irrigation efficiency. Genetic algorithms technique was applied to calibrate membership function of fuzzy model. The fuzzy sets technique considered the uncertainty of the available water resource and required area as the input variables. The approach model was applied to determine the fluctuated irrigation efficiency of the Nong Wei Irrigation Project (in the Northeast region of Thailand). Results found that the fuzzy-GAs model can be used to obtain the irrigation efficiencies, given the total available water resources and requested irrigation-area. The GAs calibration provided the optimal condition of the proposed model. Moreover, the proposed approach can be given the irrigation efficiencies which are close to the actual irrigation efficiency. Further, the results indicate that the varied irrigation efficiency is more precise than the constant irrigation efficiency.