Using fog data and weather observation data around Beijing,Tianji n,Tanggu stations from 1949 to 2002, the characteristic and weather conditions o f fog over JingJinTang expressway are analyzed. Results show that the annual mea n fog days are between 15 and 19 days along the JingJinTang expressway. The int erannual changes of fog between Beijing and Tianjin stations are same. But th ere is a little downtrend for fog days in Beijing, while a little uptrend in Tia nJin and Tanggu. The fog appears easily from wee hours to the time around sunris e, and disappers gradually after sunrise over JingJinTang expressway . There is an index degression correlation between the duration of fog and time. The weather factors such as the surface temperature, relative humidity, wind sp eed and so on have good indications for fog forecasting over the JingJinTang exp ressway. When the surface temperature is between -5℃ and 5℃, the wind speed is less than 4m?s-1 and the relative humidity is between 90% and 100%, fog appears more easily.