Abstract: Conventional methods using frequency response functions are effective to detect single damage, but are difficult to identify multiple damage locations and extents. In order to solve this problem, a two-stage method using incomplete frequency response functions is presented. The damage detection strategy is to localize the damage locations by using damage index of different nodes or degree-of-freedoms, which can be obtained from measured incomplete frequency response functions. A damage quantification formula is then deduced from the reduced degree-of-freedom system to calculate damage extents. The simulation results demonstrate the excellent performance of the proposed two-stage method based on degree-of-freedoms damage index to detect multiple damages as compared with other methods.