Re-oxidation is an important thermal process to minimize oxygen vacancies and produce high reliable Ni-MLCCs. The re-oxidation of these devices is then investigated with a series of “in-situ” impedance measurements between 400 and 500 °C in air. From the relative impedance change, chemical diffusion coefficients, associated activation energy and effective equivalent circuit model are determined. Those values were found to be reasonable compared with previous researchers’ data. Moreover, the proposed effective equivalent circuit model successfully represents the real Ni-MLCC morphology. From transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS), it is found that the electrical properties and reliabilities of the Ni-MLCCs re-oxidized under different conditions are identical.