Differentiation of stand individuals results from the competition of limited resources and thus affects allometry and allocation of tree biomass, but the extent to its impact has rarely been quantified. In this study, 38 Larix gmelinii trees, including 15 dominant, 16 intermediate, and 7 suppressed trees that were classified using a relative diameter method, were harvested for quantifying the effect of tree differentiation in the stand on the allometry and allocation of biomass. The results showed that the diameter at crown base was the most reliable predictor for branch and foliage biomass, whereas the diameter at breast height (DBH) was the best predictor for the other biomass components. For a specific value within a certain range of the DBH, the allometric models for belowground biomass components did not differ significantly among the tree differentiation classes (P > 0.05). However, the dominant trees allocated more biomass to branch and foliage components, whereas the intermediate trees allocated more biomass to stem component than the dominant, and more biomass to aboveground component than the suppressed. The tree height of the suppressed and intermediate trees was significantly greater than the dominant for a given DBH. The proportions of all the components except for stump to the total biomass did not differ significantly (P > 0.05) among the tree differentiation classes. The root shoot ratio was relatively constant for all the tree-differentiation classes. These results suggest that the differentiation of stand individuals in the larch trees mainly caused by resource competition diverts the allometry and allocation of the aboveground biomass components, but the relative allocation pattern is conservative.