Abstract: An advection fog on 14 Jan 2006 in Beijing area is simulated and diagnosed with WRF(Weather Research Forecasting)nonhydrostatic model.It shows that the advection fog was formed when warm and moist air lied on the SE,E and S of Beijing area moved to the cooler ground.The continues and weak southern wind was one of the factors that the fog could maintain.The other important reason is that inversion layer were lifted from the ground to lower level of the air and maintained.The cold air intrusion resulted in the fog dispersion.Potential ability of prediction the advection fog by WRF model is proved to some extent.