Among the various components of vegetation, root system plays an important role in stabilizing, soil reinforcement and preventing erosion. This study aimed to investigate the effect of oak decline phenomenon on root mechanical characteristics of Brant`s oak (Quercus brantii Lindl.) in Dinarkuh forests, Ilam province. Considering DBH distribution of the trees in the study site, five trees were randomly selected in small DBH (15 to 35 cm) and large DBH (35 to 55 cm) classes within three treatments of healthy, moderate decline, and severe decline. Root samples were collected from each tree by digging near the tree and then tensile test were performed in the laboratory. The results showed that the root diameter-force strength relation was positive power-law and the root diameter-tensile strength relationship was negative power-law. Results of the covariance analysis for tensile force and strength showed that there is no significant difference between tensile force and strength among treatments of small DBH class, while in large DBH class, root tensile force and strength of healthy trees were significantly higher than trees with severe decline. Based on the decline, tensile force of the trees in the treatment of severe decline compared to the healthy trees, so force decreased by 11.3% in the small DBH and 22.7% in the large DBH classes. Also, these values ​​for tensile strength showed a decrease of 13.4% and 19.7%, respectively. The findings of this study show how the effect of forest decline on the root tensile force and strength values ​​leads to a better knowledge of the controlling soil erosion in the Zagros ecosystems.