The possibility and the reliability of using Landsat TM 7 bands' data only to estimate monthly evapotranspiration (ET) over catchment areas were investigated through the regression analysis between the values of ET calculated from Morton or Penman equation and the data from satellites. The results for the Tokoro and the Abashiri River basin clearly indicated that such an approach was possible and reliable for grassland, forest and water area, and of the total 7 bands examined, four bands which belong to the visible light, near infrared, intermediate infrared and thermal infrared zone were most contributive and important, in the estimation of ET by using Landsat TM data only.