The long-term performance degradation of SOFCs is due to instability of the cell components. Because SOFCs have a composite-solid structure, thermal cycling may also cause performance degradation. In this study, the degradation of the cell performance due to thermal cycles and due to aging is examined by the AC impedance method. A specially designed electrochemical method is used to study the cell degradation due to aging. A large positive overpotential is applied at the cathode in order to accelerate the performance degradation. The AC impedance response helps to understand how the aging affects the electrode structure and may lead to performance degradation. The impedance responses generated in this study also help to understand the physical meaning of various features in the impedance diagram, and to clarify some controversial questions in terms of microstructure change in the electrode.