Abstract:The tropical rain forest (TRF) in Yunnan is similar to the rain forest of tropical Asia in floristic composition, forest profile, physiognomy and species richness, and is a particular type of the latter. It occurs at the northern margin of tropical Asia and is affected not only by the seasonal dryness of the monsoon climate, but also by the low temperature found at the relatively higher latitude and altitude. The flora of the TRF of Yunnan consists mainly of tropical floristic elements, which contribute more than 90% at the generic level and more than 80% at the specific level to its total flora. The dominant geographical elements are the tropical Asian distribution, which contributes about 40% of the genera and 70% of the species. Most of the dominant families, in terms of both numbers of species and tree importance values, are also similar to the Indo-Malesian rain forests. The formations of the TRF show clear floristic divergences between southwestern, southern and southeastern Yunnan because of the different floristic origins and evolutionary histories of these regions, although they are similar in ecological and physiognomic features. The TRF of Yunnan is maintained by the southwestern Asian monsoon. Uplift of the Himalayas triggered and strengthened the southwestern Asian monsoon climate, which created local wet habitats in tropical areas of Yunnan, and the tropical rain forest occurred locally. Based on the geological history and the palaeobotanical research in tropical Yunnan and adjacent regions, we suggest that a much drier climate prevailed in the later Tertiary or Quaternary periods, because there are drought-tolerant deciduous trees in the TRF and deciduous forests in the same areas as the TRF. The present TRF in Yunnan is an ecotone between tropical lowland and montane forests, and controlled mainly by local habitat factors.