Abstract: Considering the contribution of out-of-plane stresses and strains to laminate structural modal damping in FEM with fewer elements in the thickness direction，a composite structural modal damping prediction theory is presented based on the 3-D effective damping matrix. The composite structure was divided into several sub laminates along the thickness，and each sub laminate contains many laminas. These sub laminates'stiffness matrices，compliance matrices and 3-D effective damping matrices were calculated by the synthesizing theory. The finite elements were generated with these sub laminates，and a structural modal analysis was executed. Combining the 3-D effective damping matrices and the results of the modal analysis，specific damping capacities for each modal were calculated. The presented method is validated by the calculations for structural specific damping capacities of unidirectional laminates，symmetrical laminates and thick laminates. This method combines the accuracy and efficiency with the consideration of the contribution of out-of-plane stresses and strains.