Generally speaking, additional strength created by roots in soil is defined as growing cohesion (CR). The roots of trees increase shear strength of soil by transmitting developed shear stress to tension strength in soil texture. In other words, soil rapture leads to extension of roots. Friction between roots and soil from one hand, and limiting stresses from other hand locks roots and soil together. The result is an increase in soil strength. Additional vertical stresses and larger roots area of soil cause more limiting stresses and cohesion between root and soil, which finally leads to an increase of shear strength of soil. In this research, an attempt has been made to study effects of willow root. For this purpose, large-scale in-situ direct shear strength tests (with 50x50cm dimensions) have been performed on both rootless and rooted soils. After each test, in addition to measurement of soil shear strength, the numbers and diameters of roots have been counted and evaluated. The result shows that soil shear strength for fixed vertical loads increases with higher root density percentage. Keywords: Ratio of root area (root density), Shear zone, Root of tree and shear strength.