To explore biomass dynamics and its allocation in Eucalyptus urophylla×E. grandis plantations at different stand ages (1, 2, 3, 5, 8a), fifteen plots with an area of 20 m×50 m each, were constructed in Guangxi, southwest China. The biomass of single variable regression equation for DBH (diameter at breast height) was established based the data of 18 sample trees of E. urophylla×E. grandis with different ages and DBH. Regression analysis of sample trees in arbor layer and harvest method in quadrats for shrubs, herbs and litterfall layers, were applied to calculate the biomass of Eucalyptus urophylla × E. grandis plantations at different ages, and to analyze its composition, distribution patterns, and variation trend. The results showed that the total biomass of the plantations increased with stand age, being 12.94 t/hm² in 1a stand, 47.75 t/hm² in 2a stand, 64.51 t/hm² in 3a stand, 105.77 t/hm² in 5a stand, 137.51 t/hm² in 8a stand, with the living biomass and litter accounted for 85.60% to 97.61%, and 2.39% to 14.40%, respectively, for the five stages. Arbor layer dominated the E. urophylla × E. grandis plantation responsible for from 54.80% to 91.56% of the total biomass and the dominance increased with stand age. The litter layer was ranked the second in plantation biomass. The biomass of shrub and herb layers was very low, responsible for 1.02% to 6.47% and 0.28% to 24.33%, respectively, and the ratios decreased with stand age. In the arbor layer, trunk accounted for 51.07% to 98.48% of the biomass, and the ratios increased along with stand age, while branches, leaves, and roots were responsible for 5.76% to 11.80%, 2.17% to 21.01%, 6.72% to 14.87%, respectively, and the ratios of all the compartments decreased with stand age. In the shrub layer, branch accounted for the highest proportion (37.89% to 56.79%) of the total biomass while leaves and roots accounted for 16.35% to 34.24% and 19.52% to 39.52%, respectively, with little variations among stand ages. In the grass layer, the aboveground biomass was larger than the underground biomass from 1a stand to 5a stand, while the underground biomass accounted for 63.87% of the total biomass for the 8a stage. The total biomass and biomass of different compartments for E. urophylla × E. grandis plantations had good growth models with the total biomass increase represented by Y=-1.693×10⁴+3.337×10⁴X-1.761X². Compared with other forests, the total biomass of 8a E. urophylla × E. grandis plantation was equal to the 32 a Phoebe bourmei plantation, while lower than the tropical rain forests. However, the net annual biomass increment of E. urophylla × E. grandis plantation was 17.19 t hm^-2 a^-1, which indicated that E. urophylla × E. grandis was a fast-growing forest tree species with high photosynthesis efficiency and carbon sequestration capacity.