Using the observation data from 59 stations from 1961 to 2006 in Jiangsu Province，an analysis is made of the annual variation characteristics of heavy fog days.A further research on the generation, dissipation, and duration variation characteristics of heavy fog is conducted by means of the data from 8 stations from 1961 to 2006 The results show that the number of annual fog days increased from 1961, reached the peak in the early 1980s, and declined fast afterward; the heavy duration showed a marked increase trend, mainly the postponement of fog dissipation. A compositive impact index, including the effect of fog days and fog duration, is designed to reflect the fact that highimpact heavy fogs become more serious around the Jiangsu region.