The accuracy of a calibration of a water-vapour channel of a radiometer on board a satellite by using a radiative transfer code with radiosonde profiles was examined with the aid of the calibrated WV channel data of the Visible and Thermal Infrared Radiometer (VTIR) on the MOS-1 satellite. It has been verified that the assumption that a linear extrapolation for the relative humidity in the upper troposphere was reasonable for estimating the radiances in the water-vapour channel by a comparison of the brightness temperatures of the channel measured by the calibrated VTIR with those calculated by the radiative transfer code. An error in water-vapour distribution or radiosonde data considerably affected the estimated brightness temperature by the radiance calculation in the water vapour absorption band. The accuracy of the calibration of the water-vapour channel by the radiance calculation was with the root-mean-square (rms) temperature difference of 3–96°C.