The tropical rain forest in Xishuangbanna, SW China has a high floristic diversity and is closely related to Malaysian rain forests in flora. This forest would not normally be established in such a climatic region as Xishuangbanna (less precipitation and lower air temperature) compared to those of the lowland moist tropics. The mean annual rainfall is 1487 mm, which is considerably lower than rain forests in other parts of the world. It is believed that the frequent occurrence of radiation fog might play an important role in the water relations of plants and in the hydrological cycle of this type of rain forest. However, the multiple hydrological and ecological effects of radiation fog are not well understood. In this paper, we describe and analyze the significance of radiation fog to this forest, and develop a hypothesis that fog plays an important role in the presence of the tropical rain forest in Xishuangbanna. Suggestions for further research on the significance of fog are offered.