Abstract: In order to estimate the structural damage more accurately, a new indicator is developed to detect the damage of 2D structures, which is according to the changes in the curvature of flexibility before and after damage occurring. Numerical simulations on 2D structures are carried out for damage diagnosis with different types of indicator. Comparison between results from different indicators shows that the present indicator, with less working effort involved, generates the same value as that of the change in modal flexibility curvature. However, in 3D structures, some indicators may be missed. To deal with this problem, the sum of the change in flexibility curvatures is proposed as a diagnosis indicator. Examples of a 3D truss bridge and a plate structure show that the proposed indicator is effective in 3D structures.