In this paper,the characteristics of the spatial and temporal distribution of dew the formation mechanism of dew and the relation of dew to fog and inversion in the Xishuangbanna area are studied. In the meantime the agroclimatic significance of dew is analyzed. The results indicate that the annual dew day in the Xishuangbanna area may reach about 300 d and the maximum number of dew day in some year exceeds 340 d. The number of dew day assumes such a characteristic,namely,it is greater in dry season fromNov. to May than in rainy season from Jun. to Oct. The formation of dew in the area is mostly in the nightfall. At this time the radiation of the ground surface is strong and the temperature of the ground vegetation is lower than the dew point temperature of the air which it contacts. Thus the water vapour condenses into dew. The formation of dew not only weakens the inversion intensity near the ground but also partly reduces the formation possibility of the radiation fog. The occurrence of dew in the dry season is of great ecological importance to the growth of tropical plants in the area. It may partly miligate the rainfall insufficiency in the dry season,thus avoiding or weakening the occurrence of frostbite.