Reference Evapotranspiration (ET0) is a basic parameter for determination of irrigation program. It is one of the most important factors in water resources management and considers as a requirement for every irrigation and drainage plans. Because of the spatial variations of ET0, it is necessary to use from intrapolation methods in order to estimation of this parameter in regional studie. Using geostatistical methods is one of the ways to estimation of evapotranspiration in locations which have no synoptic stations. Determining the most important geostatistical method for spatial estimation of ET0 and its zoning in Bakhtegan basin is the main aim of this research. Therefore, recorded daily data over 2013 to 2014 yearsin 24 synoptic stations which located inthe around and the inside of Bakhtegan basin were used. The amount of daily reference evapotranspiration was determined by using the ET0-calculator software. Calculated ET0 values from geographic coordinates of synoptic stations was processed by Arc GIS software. Different geostatistical methods including Inverse Distance Weighting (IDW), ordinary kriging and co-kriging (spherical, exponential and Gaussian variogram models) were used in order to spatial estimation of ET0. On the other hand, three criteria including Root MeanSquare Error (RMSE), Mean Absolute Error (MAE) and Mean Biased Error (MBE) were used for selecting the most appropriateinterpolation method. Based upon the results of this research, co-kriging method showed the minimum RMSE, MAE and MBE equal to 0.6973, 0.5146 and -0.1155, respectively. Therefore, it was selected as the most appropriate method in estimation of spatial evapotranspiration. پرونده مقاله