In the electro-mechanical impedance technique (EMI) for structural health monitoring, the admittance change between the free state and bonded state can play a crucial role in the selection of the optimal frequency range prior to attachment. However, the frequency shift after bonding has so far been ignored. This paper investigates the change in the admittance of the PZT patch after bonding to different materials via experimentation and numerical simulation. The results show that the frequency shift of the first resonance frequency is dependent on the material of the substrate. For passive materials, the frequency shift is between −15 and 15 kHz. With active materials, the frequency shift is between 40 and 80 kHz. Furthermore, the frequency shift is related to the elastic modulus of the materials, with a high elastic modulus resulting in a large frequency shift. The findings lay the foundation for a more comprehensive understanding of the admittance at the bonded state and for the proper application of the EMI technique.