In order to understand soil reinforcement mechanisms of nitrogen-fixing plants,we chose five Leucaena trees with similar ages in Jiangjia Ravine of Dongchuan area of Yunnan Province,near which soil samples were collected on profiles of 2 m deep and 1 m wide.Soil and roots of Leucaena trees were taken for tri-axial experiments to determine the relationship among shear strength,cohesion,friction angle,root amount,and root diameter.It was found that both shear strength and cohesion related positively to the amounts and diameters of the roots.Meanwhile,the amounts and diameters of the roots negatively related to friction angles,to less extent.The results of tri-axial experiments agreed well with the cohesion principles.The cohesion of soil was enhanced by the roots;however,the friction angle of soil was degraded at the same time.According to the quantitative assessment on the roots of Leucaena trees in improving shear strength of soil,more attention should be paid to applying biological engineering measures to ecological restoration of this region.