Geographic Information Systems (GIS) are computer based tools for mapping and analysing features and events on earth. GIS technology integrates common database operations such as query and statistical analysis with the unique visualisation and geographic analysis benefits offered by maps Evapotranspiration mapping with classical methods may take months and even years based on the size of the area to be mapped. However, recently developed methods decreased the time consumed for such mapping practices to minutes. Geostatistical methods are the most commonly used methods for mapping over large areas in a short time. In present study, Reference Evapotranspiration (ETo) values were calculated by Standardize FAO Penman Montehit method using long-term climate data in the Gediz basin. The calculated ETo values were mapped for April, May, June, July, August, September and yearly total by using geoistitic method. It was used 6 station in basin and 8 station out of basins for calculation. In the production of geostatistics maps, cokriging was used as interpolation method and spherical model was used for model of semivariogram. At the end of the study, it was determined that ETo maps could be produced with reliable method and model.