Abstract: In connection with diffused damage in composite laminates, a synergistic damage mechanic model was proposed for the stiffness properties. The model included the microcosmic responses of the physical damage and macroscopic performance of the material's stiffness. In micro-level, mesoscopic RVE (representative volume element) model was established to obtain crack opening displacement and crack sliding displacement, which were used to define the damage tensor. In macro-level, through homogenizing the material strain and the surface displacement of the damage, the relationship of the stiffness matrix of unidirectional laminate or laminates in damage statue and damage tense was set up. As matrix crack for example, the constitutive relations of the laminates with transverse and longitudinal cracks were constructed, respectively. The influences of the transverse matrix cracks on the stiffness properties of the laminates ±θ/90_4S were analyzed with the present model and showed that it is capable to predict the reduction of the stiffness properties resulted from the damage in the laminates.