By using the NCEP 1°×1° grid point material and the convention observed data, a heavy fog process in the mountainous region occurred in KunmingDaluo high w ay in Yunnan in the end of December 2005 was diagnostically analyzed. The result shows that this fog ever happened at the southwest side of strong cold high pre ssure of ground in the weather background when the continuous and incessant rain stopped and turned to clear day. The ground radiation cooling effect trigge red and strengthened the mountainous region fog, and the topography leeward slop e obviously strengthened the effect on the fog of mountain region. When fog form ed, 850～800hPa level has an obvious inversion layer or the neutrosphere. The st ronger the inversion layer, the heavier the mountainous region fog. The breeze in the surface layer is advantageous in forming the fog.