Abstract: Fog presents a severe hazard in areas of intense traffic, such as airports and highways. Moreover, because air pollutants are not easy to be dispersed in foggy days, it is also harmful to human health and agricultural production. Fog prediction is essential to public safety and has a high economic value. To precisely predict foggy weather, it is essential to first understand the mechanisms responsible for fog formation and maintenance.The NCAR/PSU MM5 V37 is used to simulate and diagnose a dense fog event in Jiangsu Province and the surrounding areas during December 24—27, 2006. In the control simulation, the Gayno Seaman parameterization of the boundary layer is used. The deep convection parameterization of Grell is adopted, and the explicit treatment of warm rain is employed. To account for the important role that longwave and shortwane radiation play in fog formation, the cloud radiative scheme is employed. The center of the simulation area is set in Nanjing(32.03°N, 118.46°E). Two computational grids with horizontal grid distances of 30 and 10 km and domain sizes of 60×60 and 76×76 grid points are considered respectively in a two-way nesting method. High resolution in the boundary layer(another 9 levels joins below 200 m)is provided by thirty-two vertical levels, stretched monotonically from the surface to 100 hPa. Sensitivity experiments are performed to under stand the role of radiation. In the sensitivity tests, a series of simulations are performed with different radiative schemes, while keeping the others physic schemes fixed. The results show that longwave radiative cooling from the Earth's surface and the atmosphere are the most important factors for the formation and development of this fog event. Meanwhile, the steady atmospheric stratification and plenty moisture supply also play an important role. The fog dissipation is influenced mostly by the shortwave radiative heating and turbulent heat transfer after sunrise. During the development and maintenance stages of the fog, the surface layer is basically weak convergence region of water vapor, while in the weakening and dissipating period, the majority of the fog area is weak divergence region of water vapor. It indicates that the wide area of divergence descending accompanied with surface cooling and cold advection are the favorable conditions for the formation of a temperature inverse layer near the ground which is responsible for the maintenance of this prolonged fog event.