Tropical montane rain forest (TMR), one of the main forest vegetation types in Xishuangbanna of Yunnan Province, occurs horizontally in the north (TMRF I) and vertically in the south (TMRF II). In order to understand its tree species diversity in different zones, six fixed plots (50 m x 50 m) at different sites in Xishuangbanna were established, two (plots 1 and 2) for TMRF I, and four (plots 3, 4, 5 and 6) for TMRF II. Grid method (10 m x 10 m) was used to record all the individuals with DBH greater than 2.0 cm in each plot, and tree species richness and diversity characteristics along DBH class, as well as changing trend of tree species diversity with increasing sampling area were analyzed. The results showed that the tree stem and species richness in the six plots both declined constantly along the DBH class gradient. For the communities of TMRF I (altitude was 850 1000 m), all indices had no clear trend with the gradient, while for the communities of TMRF II (altitude 1 200 - 2000 m), Shannon-Wiener and Simpson indices declined constantly with gradient, but Pielou's evenness index had the opposite trend. In smaller DBH classes (> or =2 cm, > or =5 cm and > or =10 cm), the tree species richness, diversity, and evenness of the communities of TMRF II were obviously higher than those of the communities of TMRF I, but there was no difference between the communities of TMRF I and TMRF II in larger DBH classes (> or = 20.0 cm, > or =30.0 cm and > or =50.0 cm) . With sampling area increased, the tree species richness, Shannon-Wiener index, Simpson index and Pielou's evenness index of the six plots all had the trend to be smooth when the sampling area increased to 2 000 m2.