Abstract: Biomass estimation model for single variable (square of high tree diameter multiply by height at breast, D2H) was established based on 15oak wood of different age, size order and 15pieces of sample survey data of 1000m2at five different forest ages. We obtained oak biomass of different forest ages by using regression analysis (tree layer) and the method of quadrat harvest (shrub layer, herb layer, litter on the ground), and analyzed the composition, distribution characteristics and change tendency of different forest age biomass. The results showed that the oak biomass increased with forest age and biomass of five different age were 73.67Mg/hm2,7.47Mg/hm2,9.93Mg/hm2,9.90Mg/hm2and 200.65Mg/hm2respectively. In total biomass, living plants accounted more than 95.58% and that of litter less than 4.42%. As for level distribution of biomass, the total biomass was dominated tree layer (accounting for 93.66% - 98.68% of total biomass). Then, it was followed by litter on the ground (accounting for 1.02% - 4.42%), and the shrub layer and herb layer biomass, which accounting for 0.20% - 2.13% and 0.03% - 2.13% respectively. As for organ allocation at tree layer, stem accounted for 46.64% - 80.78% of oak biomass and increases with forest age, while branch, leaf, root for 11.61%- 36.80% , 1.00%- 4.85% and 6.61% - 11.71% respectively. At the shrub layer, branch was highest, which accounting for 32.50%- 69.07% of shrub biomass, and leaf and root accounted for 12.89% - 25.00% and 18.04% - 42.50% respectively. As for herb layer, the biomass was inversed for forest age and above biomass was greater than that of underground. With the increasing of forest age, litter biomass showed the change trend of up - down - up. Briefly, it was implied that the biomass and allocation pattern of Oak could be affected by stand ages in west of Guangxi, China.