The outgoing longwave radiation (Rlu) for estimation of evapotranspiration is essential to understand energy balance of earth. However, the ground measurement based Rlu has a limitation that the observation can just stand for the exact site, not for an area. In this study, remote sensing technique is adopted to compensate the limitation of ground observation using the geostationary satellite. We calculated Rlu using Communication, Ocean and Meteorological Satellite (COMS). We validated Rlu from COMS with Cheongmicheon (CFK) and Sulmacheon (SMK) flux tower observations controlled by Hydrological Survey Center. The results showed that Rlu from COMS represented reasonable correlation with ground based measurement. Based on the results in this study, COMS will be able to be used for estimation of evapotranspiration.