Abstract: In order to reveal the effect of classification index difference on biomass allocation, the biomass in different classes was studied after the Pinus yunnanensis seedlings were divided into 3 different classes by means of plus or minus standard deviations. The results showed that the biomass allocation of P. yunnanensis seedlings exist significant differences among 3 different classes (P<0.05) grading by ground diameter or height, and biomass of individual plant was characterized by:Ⅰ>Ⅱ>Ⅲ. The biomass allocation of each organ of seedling was ordered by leaf > stem > root. There was no significant difference in root biomass allocation ratio (P>0.05), but class Ⅰ seedlings allocated more biomass to stems, and class Ⅲ seedlings allocated more biomass to leaves. The biomass estimation model of each seedling was constructed, the optimal biomass models for different classes of P. yunnanensis seedlings were mostly power function, followed by quadratic equations after grading by ground diameter. But grading by height were all quadratic equations, and the regression relationship reached an extremely significant level (P<0.01). In conclusion, the biomass allocation of P. yunnanensis seedlings was restricted by seedling class, grading by ground diameter or height had little effect on biomass. Therefore, it is feasible to adopt ground diameter or height grading in production practice.