Abstract: The composite structures with a viscoelastic mid-layer damping materials have the characteristics of higher strength, weaker vibration and lower noise. The first-order shear deformation theory is used to describe the deformation of the structures. The governing equations are deduced by the Hamilton principle. The double Fourier series which satisfy the boundary conditions are acquired to express the vibration mode solution for the four simply supported composite structures. The results of the dynamic response of structures, the natural frequencies and loss factors are obtained. Improved genetic algorithm is used for the optimal design of the loss factor of viscoelastic composite structures. Numerical results show that the loss factor of the viscoelastic composite structures is significantly increased by the optimal design of fiber angle θ, damping layer thickness and the layer thickness ratio λ, and modulus ratios G₂. The damping properties of the structures are thus improved and the goals are realized to reduce vibration and weaken its noise.