Using the fouty years data of fog in Xishuangbanna, the following fact is discovered: the days,hours and amount (water) of fog are rapidly decreasing. For example,in Jinghong,the averaged fog hours per year from 1954 to 1960 is 858. 01 hours, but it is only 213. 56 hours from 1991 to 1995. The cause for fog decrease is the increasing of air temperature, the decreasing of the precipitation and then the lowering of relative humidity. It is pointed out that the reduced vegetation and the enhanced city-island effect are the artificial cause of fog decrease in Xishuangbanna region.