Stabilization of river terraces and natural soil slopes in rangelands by means of tree roots as a reinforcement agent is an environmental adopted method which needs to be developed. In this research a series of tests was carried out on soil blocks containing willow roots and without roots using a large scale in situ direct shear test apparatus. The results were analyzed based on direct shear theory to calculate the cohesion and internal friction angle of rooted and non-rooted soil. The results reveal that in spite of a slight decrease of 8% in the internal friction angle, the appearance cohesion of lean clay soils increase significantly up to 130%. The results of this research are presented in forms of linear and logarithmic relations between increased shear strength and root density at different depths of soil.