Abstract: In terms of microscopic textures,mineral particles are either cohesive or separated,and the cohesive force and internal frictional force of the Coulomb criterion cannot exit together locally.Cracks are produced and increase gradually with deformation,and the cohesion between particles in material loses with the deformation.The new cracks will produce friction and the shear stress of the material is supported by friction.If the loss of cohesive forces reduces the supporting capacity,then yield and failure will occur in localized fracture surfaces of a specimen and show the brittle character,otherwise the yield and failure will be distributed and show the ductile character.With increasing confining pressures,the frictional force of fissures will exceed the cohesive force of rock material.Then when the axial stress increases to the value of the cohesive force,shear yield will occur in material,producing plastic deformation.The frictional force will not increase to its maximum but keep the value of cohesive forces and the fissures will not slide either.Knowledge of the concept of friction is the key to understanding the deformation,strength and failure of various kinds of rock with changes in confining pressure.