Abstract: The biomass and its allocation of a tropical wet seasonal rain forest in Xishuangbanna were studied based on the standard tree regression analysis (for trees and lianas) and clear cut method (for shrub and herb). The total biomass of the community was 360.909t·hm-2,and its allocation in different layers was: tree layer 352.563t·hm-2 (account for 97.69%),shrub layer 4.737t·hm-2(1.31%) ,liana 3.108t·hm-2(0.86%) and herb layer 0.501t·hm-2(0.14%). Most of the biomass of the community concentrated on the tree layer, the organic allocation of the biomass in tree layer was: stem 241.27t·hm-2(68.43%) ,root 69.614t·hm-2(19.75%) ,branch 37.287 (10.57 %) t· hm-2, and leaves 4.392t·hm-2 (1.25 % ); The DBH class allocation of the biomass mainly concentrated on the middle DBH class ,the biomass of six DBH classes from 20cm to 80cm reached 255.460t·hm-2(72.46 %);The vertical allocation of the biomass in tree layer was: the sub-layer Ⅰ 219.365t·hm-2(62.22%),the sub-layer Ⅱ107.743t·hm-2(30.56%), and the sublayer Ⅲ 25.455t·hm-2 (7.22%);There are twenty-six species with biomass over 0.5% of the total biomass of the tree layer, and three species with biomass over 5% ,i.. e. , Pometia tomentosa (account for 19.67% ), Barringtonia macrostachya (5.44% ) and Terminalia myriocarpa (5.27 % ). The species biomass allocation showed that the community had distinct dominant species. The LAI of the tree layer was 5. 724.