This paper presents a novel method to monitor the bonding condition of active fiber composites (AFCs). AFCs can be used as integrated acoustic sensors/actuators to compose functional structures because of their excellent properties. Debonds between AFC patch and host structure should be avoided and surveyed through its service life. A partially debonded patch bears an axial extensional vibration which is excited by a sinusoidal voltage and detected by the interdigital electrodes symmetrically aligned on opposite surfaces of the patch. The electric impedance and mechanical displacement of the AFC patch adhered on an aluminum plate were investigated in a broad frequency range. The modal characteristics depend on the size of debond and stiffness of adhesive in front of the edge of delamination. The debonding ratio of the AFC patch is in inverse proportion to the resonant frequency of the fundamental mode. Feasibility of self-detecting the progressive delamination between AFC patch and host plate is demonstrated through computational and experimental results.