Abstract: The Microwave Humidity Sounder (MWHS) is a five-channel microwave radiometer in the range of 150～191 GHz onboard FengYun 3 (FY-3). Before the launch of FY-3A, the intensive thermal vacuum (TVAC) tests for MWHS had been carried out in a 2-m TVAC chamber, and the basic parameters such as receiver nonlinearity were obtained. Four-year operation shows that the performance of FY-3A MWHS remains stabile. The variations among space views and warm target views in the interval of scan lines were generally within 20 counts. In addition, the temperature fl uctuation of the warm target in a single pass is within 0.5 K. Primarily, due to the nonlinearity correction obtained from the TVAC test, radiance measurements of MWHS agree well with NOAA-17 AMSU-B and NOAA-18 Microwave Humidity Sounder (MHS) data. Averaged brightness temperature differences between FY-3 MWHS and NOAA-17 AMSU-B, at simultaneous cross-overpass points, are less than 1.08 K for the channels at 183±1 GHz. In the observation-minus-background comparison, the observed and simulated brightness temperature biases of FY-3 MWHS and NOAA-18 MHS exhibit similar performance in the 183.31±1 GHz channel; but in the 183.31±7 GHz channel, MWHS agrees better with background. It is anticipated that FY-3 MWHS data will contribute to numerical weather prediction.