The ground-based microwave sounding radiometers installed at 9 weather stations of Korea Meteorological Administration alongside with the wind profilers have been operated for more than 4 years. Here we introduce a process to assess the characteristics of the instrument calibration by comparing the measured brightnesstemperature (Tb) with the theoretical reference data, which are prepared by the radiative transfer simulation with the temperature and humidity profiles from the numerical weather prediction model. Based on the three years of data, from 2010 to 2012, we were able to characterize the eects of the absolute calibration, the thick clouds, and the frequency calibration to the quality of the measured Tb. When the three eects are properly considered, including the frequency adjustment which is estimated using the simulated Tb, the measured and simulated Tb show an excellent agreement. The regression coe-cients are better than 0.97 along with the bias value of better than 0.5 K. However, the variability given as the SD of dierence between the measured and simulated Tb, show a relatively large value at the lower observation frequencies, as large as 2.6 K at the 51.28 GHz channel, while they improve with the increasing frequency.