This paper presents a method for structural damage identification based on the modal data change before and after the occurrence of damage. In this method, it is assumed that the reduction of structural stiffness due to damage as the summation of each elemental stiffness matrix multiplied by a damage coefficient. And then, the damage coefficient (damage extent) can be solved adversely from the vibration equation. Furthermore, the modal assurance criterion (MAC) is introduced to check the correlation of mode shapes between the damaged and the undamaged structure. An advantage of employing the MAC is that it can analyze the sensitivity of mode shapes to damage. To demonstrate the capability of the proposed method, an example of a six-span planar truss beam is used to verify the present method numerically. Results indicate that the proposed method is effective in locating single or multiple damage locations and quantifying damage extent in the truss structure.