Heavy fog is one of the main disaster weather. From January 1 to 5,2007, a heavy fog formed over north China plain. Multipledata was used to analyses the weat her background, including temperature and humidity condition and the stratificat ion of atmosphere. The results showed that during the days of heavy fog, the syn optic situation was stable. There was no cold air outburst. The circulation situ ation of the middle and high levels in Asia and Europe was one ridge two troughs circulation pattern, and the south branch air flow was straight. North China pla in was situated at the rear of high pressure moving to the sea and there was wea k wind blowing from northeast or east. The weak wind was not only favorable to t he advection transfer of the moist and warm air, but also didn't destroy the for mation conditions of heavy fog. At the same time, the stratification of atmosphe re was absolutely stable and there was a deep inversion layer. When the moist an d warm air advected onto this cold ground, they was cooling down and forming hea vy fog. So this fog belonged to advection cooling fog. The heavy fog lasted 5 da ys because of the maintenance of the stable synoptic situation and the air advec tion. In addition, the low sea level elevation of North China plain was another important reason of the formation of heavy fog because it was favorable to the a dvection.