Based on conventional observation, the NCEP reanalysis data (1°×1°), regional automatic meteorological station, data and L band sounding data, three persistent heavy fog events lasting over 10 days across the North China Plain in 2000-2013 are analyzed. The results show that all the three sustained dense fog events occur in the background of zonal circulation. The geopotential height field, humidity field, tempereture field and sea level pressure during the events are very similar. The surface weather conditions at the beginning of and during the dense fog events are also similar. The sustained dense fogs in the North China Plain are caused by the long term static and stable synoptic pattern which is attributed to blocking function of Mountain Taihang and Mountain Yanshan to cold airs in the zonal circulation setting. Several upper short wave troughs mobiling to the North China Plain and large scale sinking movement lead to the maintenance and enhancement of fog. Topographic convergence line caused by Mountain Taihang and moderately stable inversion which is formed by west current crossing Mountain Taihang also play an important role in the enhancement and maintain of the dense fog events.