Plant has a strong influence on shear strength properties of soil. Recently, slope protection method using plant is increased in public works, thus mechanism of preventing slope failure by root is expected to be clarified. In this study, in order to clarify relationships between plant and shear strength properties of soil, shear box tests of soil with plant were conducted. The strength properties of soil focusing on the number of plant were evaluated by measuring shear stress and strength parameters of soil with plant. Furthermore, vertical displacement, horizontal displacement and bioelectric potential of stem and root were measured during shear test. The relationship between bioelectric potential of plant and shearing behaviour of soil was considered. As a result of shear box tests, when plant was existed in the area of shear box, strength parameter cd, ϕd was increased respectively compared with the area without root. Furthermore, amplitude of bioelectric potential was changed after maximum shear stress was shown.