Abstract: Biomass allocation is the most useful tool for studying plant structure and function, and plays an important role in forest ecosystem carbon cycling. In this study, we selected twelve dominant species, e.g., Betula platyphylla, Acer mandshuricum, Ulmus japonica, Pinus koraiensis, Juglans mandshurica, Maackia kiaamurensis, Quercus mongolica, Carpinus cordata, Populus ussuriensis, Acer mono, Fraxinus mandshurica and Tilia amurensis in a natural secondary mixed forest in the Administration Bureau for Jiaohe Forestry Experimental Area, Jilin Province, northeast China. Biomass partitioning of different components including leaf, branch, stem and root was investigated. Meanwhile, the allometric relationships of biomass components and tree size were developed. The main results showed that: 1) the relative growth of biomass components of all species followed the allometric theory, and the allometric power exponents of components were plastic. The MB∝MS, MB∝MR, MS∝MR and MR∝MAG were isometric, the 95% confidence interval of allometric power exponents (α) were 1.091-1.254, 1.012-1.158, 1.015-1.202 and 0.991-1.070, respectively, and the mean value of α were all approximate to the theoretical value (α=1). However, the ML∝MB, ML∝MS and ML∝MR were allometric, the 95% confidence interval of α were 0.655-0.757, 0.777-0.931 and 0.718-0.859 and the mean values of α were 0.706, 0.854 and 0.789, respectively. 2) All biomass components allocation of twelve species followed the allometric allocation theory, the biomass allocation proportion of leaf, branch, stem and root were 1.80%-6.54%, 13.87%-27.09, 51.12%-65.03% and 15.76%-25.52%, respectively. And the mean value of proportion of different biomass components showed an order of stem (57.09%) >, root (21.46%) >, branch (18.59%) >, leaf (2.86%). The root/shoot ratio for all species ranged from 0.189-0.355 with the average value of 0.279. 3) The proportion of biomass allocation of all components and root/shoot ratio were affected by tree species, and there were significant differences (P<, 0.05) among tree species in the proportion of biomass allocation and root/shoot ratio. There were no significant correlations (P>, 0.05) between tree size and allocation proportion of root biomass as well as root/shoot ratio, however, allocation proportion of leaf, branch and stem biomass was significantly correlated with tree size (P<, 0.05) . We concluded that plant organs do not always follow isometric growth in the growth process, allometric growth was instead ubiquitous in the growth process of various organs, meanwhile, the biomass allocation of plant organs follows allometric distribution theory. In order to obtain more space and nutrition, plant follows optimized distribution theory in its growth process and allocates more resources to competitive organs in order to increase its competitiveness and productivity.