Abstract: Fog is an aerosol system composed of plenty suspended tiny water or ice crystal which have severe scatter effect on visible light causing obstacle to visibilit y.There are radiative fog,advection fog and frontal fog according to the formation reasons of fog.Dayaoshan is a main mountain in Nanling Mountain area where is the important interface of climates in China.The highway from Peking to Zhuhai cross the Dayaoshan,the height above sea level of road surf ace ascend from 200 m to 800 m,and then descend to 200 m,the topogranphy is very complicated,heavy fog occurs frequently.In Winter and Spring,the visibility less 500 m is over 37% according to hourly statistics,so there are good chances to collect fog water and analyse it's chemical compositions.Based on the fog-water data from December 1998 to January 1999,and February,March 2001,the chemical compositions of fog-water in heavy fog processes have been analyzed,especially for the ion concentration of fogwater and t heir sources in three typical heavy fog processes.The analyses show the frequency of acid fog reach 50%,and the harms caused by acid fog should not be neglected.Allion concent rations of fog-water in heavy fog processes are far higher than those in rainwater.Therefore,fog not only cause bad visibility,but also harm peoples health due to its highly polluted particles.When fog occurs,the high concentration pollutedion compositions will stimulate mucous membrane of respiratory tract,causing respiratory disease.The chemical features are obviously different from different fog processes.The highest negativeion concentration in fog-water is SO42-;follow ing NO3-,while the Ca2+ and NH4+ are highest positiveion concentrations.Rain-water is more acid than fog-water,showing althoughion concentrations of fog-water are far higher than those in rain-water,there are more cushioning substances in the largeion compositions of fog-water like NH4+ and Ca2+.The Ca2+,SO42- and Mg2+ are obviously enriched.The fog-water in Nanling Mountain is mainly influenced by mainland environment and human activities.