An ensemble fog forecast system was designed based on WRF and a high resolution numerical 1-D model called PAFOG,whose initial perturbation conditions were obtained from the simulation of WRF.Comparison between prediction and observation was made during a typical radiation fog event that occurred in Nanjing during 13—14 December 2007.The results show the ensemble forecast system is better than single forecast.By using this method,the predictions for both visibility on surface and vertical structure of fog are better than those utilizing the model only initialized by radiosonde observation.The initial time of simulation played an important role in prediction accuracy.Because most radiation fog occurs at night and it takes some time for the model to stabilize,the best time to start the one-dimensional model is 14:00.The standard deviation between ensemble member and observation is 0.516 and the absolute error of ensemble average is 0.287 km.