Abstract: A microscopic model is proposed for the mechanical damage of rocks subjected to true triaxial stresses toanalyze the influence of intermdiate principal stress on the constitutive relatioriship of rocks. Both the kinetic equa-tions of damage evolution and the formulations of damage compliance are developed. The numerical results indicatethat the intermediate principal stress has a strong influence on the mechanical properties of rocks It is shown that asthe intermediate principal stress increases the strain decreases in the direction of maximum principal stress directionand increases in the direction of minimum principal stress direction respectively But, when the maximum principalstress is large, large intermediate principal stress makes rock deform difficultly in the direction of maximum principalstress and easily in the dirertion of minimum principal stress