Abstract: The biomass allocation pattern is critical for understanding individual growth processes and modeling terrestrial ecosystem carbon cycles in the context of global climate change. Our objective was to determine the effects of stand density on biomass allocation pattern in a Pinus tabuliformis plantation in Beijing, China. Eighteen sample trees for aboveground components and eleven sample trees for belowground components were used for developing DBH-biomass models by the nested regression method. Thirty-three temporary plots(20 m×30 m)with different stand densities(267-3 367 trees/hm²)were investigated by recording DBH of all trees over 5 cm DBH in July—August 2012. All components exhibited significant variations across the surveyed plots with different stand densities. Above and below ground biomass ranged from 20.74 to 141.25 t/hm² and 5.36 to 36.92 t/hm², respectively. The average biomass ratio of root to shoot was 0.276, and increased from 0.223 to 0.313 as stands becoming denser. In addition, with increasing stand density, the proportion of stem and branch to total forest biomass decreased, while foliage, fine root and coarse root biomass increased. The functional balance theory is tested in part by our results, which were also improtant for accurate estimation of ecosystem biomass and carbon accounting.