Based on the observation in the basic stations of the country and the interpolated data at these station points from the T106 model output for 1995—2000, the characteristics of the low-level atmospheric elements before and right at the fog occurrence over North China plain are analyzed. Moreover, the relationship between the elements distribution and occurrence frequency of fog is also given. The results indicate that the weak wind, relative humidity in the range of 80%—90%, the depression of dew point in the range of 2—4℃, stable or weak instable atmosphere and the surface temperature in the range of 3—9℃are favorable conditions for the occurrence of fog.