In order to estimate areal evapotranspiration (ET), a system using Landsat MSS and elevation data is developed. After extraction of the study area and its geometric correction, Landsat MSS data are classified into land use categories, which give empirical parameters for calculation of ET. Elevation data, which are originally given at the mesh of 7.5” in latitude by 11.25” in longitude, are interpolated and fitted to each pixel of land use image data. Calculation of ET is executed by using empirical functions in terms of meteorological elements which are linearly regressed with elevation from data at several observatories. The calculated results are represented by the form of not only statistical quantities but also transformed image data. The distribution of ET over mountainous areas, represented as image data, reasonably shows the effect of topography. After comparing with observed data of pan evaporation and with the values estimated by Thornthwaite's method, one can conclude that this system would reliably estimate the actual monthly ET over large areas.