An analysis is made of a frontal fog event occurred in the lowlatitude mountainous area in Guizhou Province between 21 and 22 February 2008 by using conventional observation and NCEP/NCAR reanalysis data. The results show that the frontal fog occurred under special weather. The key weather systems for frontal fog formation include: the strong southwest air current at 500 hPa, the lowlevel southwest jet, and the decreasing quasistationary front. The fog lasted from evening to next morning, and the large area of frontal fog appeared before and after the thin quasistationary front. There was inverse humidity near the front zone, little rainfall and frontal inversion over Guizhou during the development and progression of the fog. The forming of the frontal fog in Guizhou was closely related to the quasistationary front, and the special mountain terrain led to the maintenance of the frontal fog.