A diagnostic method of inspecting structural integrity using vibration, which is generated and monitored by piezoelectric transducers, is presented. When damage occurs in structures, the flexural-wave propagation characteristics change because of discontinuities in structural properties. To monitor this change, frequency-dependent variation of the wavenumber is measured from the piezoelectrically actuated harmonic vibration of a structure. The theoretical model was proposed to analyze the wave propagation and standing-wave pattern in the structure. Its prediction was compared to the measured vibration response on the basis of which the theoretical model was verified. Using the predicted response, the sensitivity of the potential energy to damage is obtained. With damages of different sizes and locations induced on the beam, the change in the wavenumber and equivalent dynamic stiffness was obtained from the measured transfer functions. The location and size of damage was identified from the damage index accurately. The correlation coefficient between the sensitivity of the potential energy and the variation of the dynamic stiffness was used in estimating the damage index. Analysis of the flexural-wave propagation from piezoelectric actuation allowed continued and precise structural health monitoring.