Based on T213 data and conventional observation data,a dense fog occurring in North China plain between 19th—21st,November,2005 was simulated by using mesoscale model MM5.Results show that the heavy fog is a radiation fog occurring in stable synoptic situation.The location,intensity,formation and dissipation of the simulated fog are in accordance with the real situation.The inversion layer and abundant moisture play an important role in the maintenance of the fog.Diagnostic analysis indicate that:the fog area locates in weak vapour convergence area;the descending movement accaused by the divergence and negative vorticity above 900 hPa contributes to the formation of the inversion layer;the cold advection is also favourable for the formation of the inversion layer;the long wave radiation cooling is a predominant factor for tempereture decresing;and the short wave radiation plays an important role in the fog dissipation and diurnal variations of the fog.