Abstract: Climatological characteristics of the spatio-temporal distribution and change trend of fog days in Hainan Province were analyzed based on the observational data of Hainan in 1969-2008, and the causes for changes in fog days were also analyzed based on the NCEP/NCAR reanalysis data and meteorological factors. The results show that the fog days was the most in the central mountainous area, the next in the northern coastland, and the least in the southern coastland; and yearly fog days exhibited a decreasing trend. Analysis also indicates that yearly fog days was negatively closely correlated with average minimum temperature and positively correlated with relative humidity. Fogs formed more frequently and successively in autumn, winter and early spring. The regions with a higher altitude or a lower temperature had more fog days. In the years when fog days was more than normal, the 850 hPa continental high was relatively weaker, and the 500 hPa subtropical high was relatively weaker too; and vice versa in less fog days years.