Abstract: The dynamic and thermal features of a sustained heavy fog event that occurred in the North China Plain on 29 Nov.～3 Dec.2004 are analyzed by using the station observation data and NCEP re-analysis data.The results show that a warm high ridge at the middle level and transformed surface cold high over the plain provide a good background condition;a cooling process produced by net surface radiation fluxes is the trigger and strengthening mechanism for the fog event;the subsidence flow at the middle and low tropospheric levels is favorable for the setting-up and maintenance of stable stratification at the boundary layer and weak wind condition at the ground layer over the fog region;the transportation of weak warm advection at lower levels and weak lift motion at the boundary layer play a very important role to the long maintenance of the fog event;the intrusion of north wind accompanied by cold advection is a main dynamic factor of the fog dissipation.