Abstract: Using the thick fog frequency data in 137 station sites for 46 years from 1956 to 2001,and on the basis of Geographic Information System mapping software,this paper runs statistical analysis of the spatial distribution of thick fog frequency,interdecadal variability and regional variability characteristics,and makes time series-related figures of fog frequency. Results have the following findings. Fog concentrated the most in southeast coastal regions and Sichuan Basin but the least in northwest region,the Qinghai-Tibet Plateau and Inner Mongolia. Fog frequency shows a pattern of two valleys at the beginning and ending of the 46-year period,and a wave trough in intermediate time series. The absolute value of fog frequency correlation coefficient is less than 0.3 in most parts of the country while the region of negative correlation coefficient is significantly more than the one of positive correlation coefficient. In addition,the fog frequency in most regions is generally decreased or little changed from 1956 to 2001,except in North China Plain.