Based on the conventional observation data, NCEP reanalysis data, and the automatic weather station data, a dense fog weather lasting from 24 to 27 in December of 2006 in Jiangsu is analyzed and the features of the maintaining and dissipation of this dense fog is diagnostically emphasized to carry out diagnostical study. Results show that the fog formed under the condition of radiation cooling, which changed from the radiation fog to advection fog and enhanced by warm advection. The warm and wet advection provided favorable water vapor and thermal conditions for development of the fog. The high-middle level convergence sinking, which cause temperature to go up and the low-level weak upward flow movement made the temperature inversion layer steady, which were the favorable dynamic conditions for the maintaining of the dense fog. At the fog dissipation stage, the atmospheric stratification became unstable potentially because of the southward positive vorticity advection. With the invasion of cold air, the temperature inversion layer was interrupted entirely by the divergence descending movement and the increasing vertical movement. The fog diminished under the coordination of both dynamic and thermal conditions.