The southeastern Tibetan Plateau has the highest timberlines in the world and climate change affects this area to a considerable extent.Quantification of vegetation biomass in the forests at the timberline is essential for understanding carbon balance,forest structure and functional changes in the forests at timberlines.We describe biomass and its constituent components of a smith fir timberline forest occurring at an elevation of 4320 m on the Sergyemla Mountain in the southeastern Tibetan Plateau.The results show that,in the tree layer,the average diameter at breast height(DBH) was 35.6 cm,the density 251 trees per hectare with a total biomass of 309.9 t.ha-1.Of the total biomass,the trunks accounted for 70.49%,branches for 9.09%,leaves for 5.57% and roots for 14.85%.The total biomass in the shrub layer was 77.9 t.ha-1 of which the biomass of trunks,branches,leaves and roots accounted for 42.8,14.9,4.6 and 15.6 t.ha-1,respectively.The total biomass in the herb layer was 3.0 t.ha-1.The total vegetation biomass in the smith fir timberline forest was 390.9 t.ha-1,to which the tree layer contributed the most(79.3%),followed by shrubs(19.9%) and herbs(0.8%).We estimate that the smith fir forest stand at the timberline had a higher total biomass than other fir or spruce forest stands in the surrounding area. The southeastern Tibetan Plateau has the highest timberlines in the world and climate change affects this area to a substantial extent. Quantification of vegetation biomass in the forests at the timberline is essential for understanding carbon balance, forest structure and functional changes in the forests at timberlines. The results show that, in the tree layer, the average diameter at breast height (DBH) was 35.6 cm, the density 251 trees per hectare with a total biomass of 309.9 t.ha-1.Of the total biomass, the trunks accounted for 70.49%, branches for 9.09%, leaves for 5.57% and roots for 14.85%. The total biomass in the shrub layer was 77.9 t.ha-1 of which the biomass of trunks, branches, leaves and roots accounted for 42.8, 14.9, 4.6 and 15.6 t.ha-1, respectively. The total biomass in the herb layer was 3.0 t.ha-1.Th e total vegetation biomass in the smith fir timberline forest was 390.9 t.ha-1, to which the tree layer contributed the most (79.3%), followed by shrubs (19.9%) and herbs (0.8%). We estimate that the smith fir forest stand at the timberline had a higher total biomass than other fir or spruce forest stands in the surrounding area.