Abstract: This paper verifies modal curvature difference method in theory at first,aiming at the feasibility of the damage recognition of structures.A matrix formula by using modal vector difference before and after damage is used to evaluate the damage factors of structures.A standard beam example is adapted to verify the proposed method.It is shown that the method is effective in diagnosing damage locations and degree.Additionally,according to two linear hypothesis of this paper,a simple and effective method is set up to evaluate the service life of structures.