The main purpose is to analysis a regional fog episode occurring in most of middle and eastern China from 17 December to 19 December 2004 based on the observation and the fifth generation Pann State/NCAR Mesoscale Model modeling. The characteristics and evolution process of the regional fog event are investigated. The results show that it is a typical radiation fog event with a large spatial scale and the horizontal distribution is not uniform. The fog appeared at the surface firstly and then expended to the high level. The burst development features of the fog are not found. The temperature inversion and abundant water vapor are the beneficial factors to the fog formation and maintenance as well as breeze condition. The influences of the shortwave radiation warming and longwave radiation cooling are examined.