A mechanical conversion model was developed using a numerical method to correlate the shear strength increment due to root, ΔS(subscript r), with ultimate pull-out resistance, P(subscript u), of the soil/root system. The conversion model can be applied to determine the required shear strength parameters for stability analyses of vegetated slopes. Trema orientalis (L.) Blume (India charcoal trema), a pioneer and dominant tree on the low slopeland of Taiwan, was selected as the target species of the study for the field investigation of root morphology, in-situ pull-out tests, and laboratory tensile strength tests. Using the root morphology and the strength parameters of the root material, one can develop a 2-D numerical model of the soil/root system to simulate ultimate pull-out resistance, P(subscript u), from the pull-out test and the shear strength increment due to roots, ΔSr, of the soil/root system from direct shear tests. Eventually, a relationship function, ΔS(subscript r)=f (P(subscript u)) can be proposed for practical use.