Abstract: Traditional structural optimization methods require predefined load conditions.The resulting structure is optimal under the given condition,but can be weak under different loads.Objects can suffer from various forces in practical applications.The overall performance of objects cannot be guaranteed and thus more material than actually needed is used.In this work we propose a novel approach to enhance the global strength of 3D objects under all possible load distribution,which make the strength of the object isotropic to resist different forces.The method is based on modal analysis.We first detect the weak region of the object and then reinforce it by optimizing the eigenvalue of the stiffness matrix.Based on the concept of Rayleigh Quotient,an efficient algorithm is also presented.Experiments show that our method can effectively improve the global strength of 3D objects.