Abstract: In the view of structure-borne sound, a diagnosis method for the detection and location of damage of beam structures by means of vibrational power flow is investigated. Damages in a structure cause changes in the physical coefficients of mass density, elastic modulus and damping coefficients. In this paper, the damage is modeled as a joint of a local spring. The damage point transfer matrix and the beam element transfer matrix are deduced, then the relations of the vibrational power flow, the position and the characteristic size of the damage are obtained combined with periodic structure theory. The damage can be diagnosed by comparing the vibrational power flow of beam structures with and without a damage. The approach presented here can sense the presence of damage, and locate and characterizethe damage to a satisfactory precision.