Abstract: The adhesion friction theory was applied to study the variation of shear area of structural plane and its influence on shear stress.The shearing failure of structural plane was considered to correspond to the maximum of real contact area.A criterion was given to distinguish the shearing failure of structural plane and a new selecting method of shear strength was brought forward subsequently.This method was applied in the direct shear tests on structural plane from the 24th section of Hurongxi Highway in Hubei Province to select the shear strength.The selected result was approximately the peak strength after multiplying a reduction factor,but the proposed method was more reasonable and more convenient than selecting a reduction factor for the peak strength.