The first lithium-ion insertion and extraction processes of a graphite electrode at different potentials were studied by electrochemical impedance spectroscopy (EIS). The results revealed that, below the potential of 0.65 V during the first lithiation, three semicircles were observed in the Nyquist plots; namely, two semicircles appeared in the middle-frequency region. This new phenomenon has been investigated through the detailed analysis of the change of kinetic parameters obtained from simulating the experimental EIS data for lithium-ion insertion and extraction in graphite as a function of potential. It has been found that the two semicircles in the middle-frequency region were strongly potential-dependent, and they were both attributed to the charge-transfer process. Evidence was presented (supported by model calculation) that the two semicircles observed in the middle-frequency region originated from the nonhomogeneous, multilayered porous microstructure of the graphite electrode, which resulted from a parallel combination of impedance response of the thinner parts and thicker parts of the electrode.