Abstract:tropical montane rain forest was investigated in Mengsong, south-west to Jinghong City ofXishuangbanna region in the south of Yunnan near the border between China and Myanmar. There are 623 species of seed plants belonging to 327 genera and 116 families in this forest. Analysis of areal types shows that 32.8% of the families are of tropical and subtropical type, 23.3% tropical to temperate type, and 18.1% typically tropical type. Areal types at generic level mostly are tropical (81.4%), in which 28.4% are of Indo-Malaysian type and 26.8% are of pantropic type. The ratio of tropical to temperate genera is 258:55. As for specific rank, the elements of tropical Asia type reaches 65.0%, and that of endemic to China 26.5%. The number of tropical species (431 species) is far more than the temperate ones (18 species). It is concluded that this flora is tropical in nature with obvious characteristics of Indo-Malaysia flora. It is a transitional area of tropics to subtropics, and is the north distribution limits for many typically tropical plants.