The purpose of this study is to complement the existing body of knowledge on the root system of tropical plants and their contribution to slope stability. For this reason, four trees of Acacia mangium selected as randomly in rainforest along East-West Highway, Malaysia. Root area ratio (RAR) and tensile strength (Tr) data were collected and analyzed. RAR data were measured with the whole root system and Tr were measured by individual roots. The results showed that there is a power law equation between root diameter and Tr, with an average of 44.11 Mpa (in average root diameter of 0.9mm). There was a wide variation in the RAR in relation to soil depth. Generally, RAR reduced with the increasing soil depth. Similarly, additional cohesion by plant roots conducted by Tr and RAR. In conclusion, in order to assess soil reinforcement by vegetation roots, root distribution and tensile strength data with soil depth in different vegetation and sites is necessary.