Abstract: The observation study of a persistent heavy fog event occurred in Huabei Plain from 29 Nov to 3 Dec in 2004 is undertaken using the dense observation data and NCEP/NCAR reanalysis data. It reveals the physics mechanism of its formation and maintenance by means of analyzing the large scale dynamic and thermal features of the event. The results show that the warm high ridge at middle and low troposphere and the persistent surface cold high over Huabei Plain provide an appropriate background condition for the prolonged dense fog. The warm high ridge is prone to the formation of the inversion temperature layer in boundary layer, and the dominant weak easterly winds in the central and south of the surface cold high transmit the water vapor from the east ocean area to the Huabei Plain. During the fog period, the surface wind velocity is about 2—4 m/s and the domineat wind directions are east and southeast. The bias between surface temperature and dew point is less than 2 ℃, but enlarges sharply when the dense fog process ends. By analyzing the atmospheric sounding data it demonstrates that the atmosphere over Huabei Plain dense fog region indicates convective stability and exists a downward current and an inversion layer above the fog level. The humid layer locates between groun level and 900 hPa, and becomes deeper in the late of the fog event. A cooling process at ground layer produced by net surface radiation fluxes can trigger and strengthen the dense fog event. The subsidence flow at middle and lower troposphere is important to the establishmnent and maintenance of stable stratification at boundary layer and is helpful to the weak breeze wind at ground layer over the fog region; the transportation of weak warm advection in boundary layer and weak lift motion at ground layer play a very important role in the long time maintenance of this fog event. The intrusion of north wind accompanied by cold advection is a main dynamic factor of the fog dissipation. It needs to be pointed out that although these conclusion are meaningful for recognizing this fog event, the physical mechanism on its happening and development remains unclear, especially the influence of the temperature advection process and the low cloud process. The Huabei Plain fog event exhibits a radiation fog in the beginning and early phase, but it shows more advection than radiation cooling in the later phase when the daily temperature variation is considerably weak. It is important to acquire more and higher resolution observation data especially the data in the boundary layer in order to deeply analyze the effect of warm temperature advection during the late dense fog event. The numerical simulation study by connection of two-dimension cloud model and meso-scale model and more dynamical diagnoses will need to be done in the near future.