In order to improve efficiency and reliability for damage detection, based on the characteristics of truss structures, the elemental modal strain can be derived from the nodal modal displacements by means of the finite element method on spatial truss elements connected by pin at elemental two nodes. Therefore it is presented as a new dynamic detective indicator using changes between an intact structure and a corresponding destructive structure in the elemental lower modal strain, and then a new method of damage localization named as change in elemental modal strain is set up for truss structures. A numerical example for a planar truss structure is provided to verify the practicability of the method only by one lower mode. The results show that this method is effective to locate not only single damage but also multiple ones, both light damages and severe ones. Furthermore, the results indicate that the proposed method based on modal strain of truss elements can accurately detect damage locations of truss structures with the noisy modal data.