In this paper a new nondestructive damage identification method is introduced. The method based on flexibility matrix can be used to detect and locate structural damage and evaluate the severity of damage in legs of jacket platforms by modal parameters of a structure. With the modal data for only the few lower modes in both the intact and damaged tes, the one-dimensional and two-dimensional distributed curvatures can be used to analyze damage location and the severity. Instead of directly comparing the curvatures before and after damage, the method here uses modal parameters only in the damaged structure to detect the damage and it consists of three parts. First, flexibility matrix is btained by use of the absolute maximum in each column. Second, because the legs of jacket platforms are the pipe-like structure, the circumferential flexibility curvature matrix is obtained by use of the circular curvature. At last, equivalent curvature ratio is defined and the curve meaning equivalent curvature ratio and the severity of damage elationship for one element is given through the data of damage severity from ten percent to ninety percent by numerical simulation. Many existing damage detection methods need two steps, locate the damage firstly and evaluate the severity of the damage. However, the method presented!in this paper can locate and then evaluate the severity of damage t the same time. The numerical analysis results indicate that the present method is effective, useful and only need the first and the second mode data of the structure.