Based on 20 years' monitoring in montane rain forest plots (1.25hm² within 5 blocks) in Xishuangbanna, we analyzed the dynamics of tree layer in terms of species composition, diversity index, changes of individual tree numbers, changes of dominant tree species, and diameter distribution. The results showed that in 20 years, the tree species numbers were increased from 215 (in 61 families and 128 genera) to 239 (in 62 families and 148 genera). The numbers of families, genera and species were slowly increased at the rates of 1.64%, 15.63% and 11.16%. Those entering into or withdrawing from the tree layer belonged to accidental species with less individuals. The total individual numbers of trees were increased from 1464 to 1478 while 620 were died with a mortality rate at 29.55%, the average annual mortality rate at 1.48%. Meanwhile, 634 trees entered into the tree layer and the total in growth rate was 30.22%, the average annual growth rate at 1.51%. The diversity index showed a slight fluctuation, with increasing rates of Simpson index, Shannon-Wiener index and Pielou's index at 0.28%, 2.69% and 0.55%, respectively. The death and advanced of different diameter classes were basically at flat state.