Solar-channel calibration of Korean geostationary satellite, Communication, Ocean and Meteorological Satellite (COMS), is assessed from three vicarious methods using cloud targets. Firstly, a ray-matching technique is used for collocated targets having the same solar and viewing geometries, while Moderate Resolution Imaging Spectroradiometer (MODIS) 0.6-mm channel serves as a reference. Secondly, collocated MODIS cloud products are used as inputs to a radiative transfer model (RTM) to produce COMS visible channel reflectances for moderately thick cloud targets. Lastly, deep convective cloud (DCC) targets are chosen based on COMS infrared brightness temperatures, and representative optical properties are used as RTM inputs for the TOA reflectance simulation. All three methods suggest a similar degree of biases around 9–10% in COMS visible reflectances.