A radiation fog on 20 November 2006 in Beijing area is simulated and diagnosed with MM5 nonhydrostatic model. By contrasting the image of the NOAA satellite with the observation data of radiometer, the results showed that the MM5 could well simulate this fog event, especially in simulating the fog area, the height and the duration of the fog. Furthermore, aiming at explicit moisture scheme, longwave radiation, shortwave radiation, and vertical resolution, four sensitivity experiments were performed. The results show that longwave radiative cooling is an important factor of the formation of the fog, while the shortwave radiative warming is an important factor of the fog dispersal. In addition, the results were obviously improved by increasing the vertical resolution or using more complicated explicit cloud scheme.