Abstract: The concept of damage variable is extended from material to members and structures in the paper. The valid elastic modulus of structure is used to describe the damage extent of each structural element. According to the change of modal strain energy in each element before and after the occurrence of damage, the damage index of each element is obtained. Considering the incompleteness of measured structural mode shapes, the practical complete modal space theory is used to eliminate the influence of modal truncation error. The method is used to locate a 6-bay plane cantilever truss damage and determine the magnitudes of damage. It is shown that the method can successfully detect structural damage and describe the damage extent.