The mechanism of the recovery of the nonlinearity voltage-current (V-I) characteristics after electrical degradation was investigated from various viewpoints. The recovery processes of ZnO varistors in various gas atmospheres, and at various pressures, temperatures, and absolute humidity levels were evaluated. The evaluation involved the analysis of V-I characteristics, capacitance-voltage (C-V) characteristics, and X-ray diffraction (XRD). The height of the Schottky barrier and the thickness of the depletion layer were almost constant for sintering times between 3 and 5 h. On the other hand, the rate of recovery exhibited a change with sintering time. The recovery speed of the nonlinearity was also temperature dependent. It is suggested that the recovery process is caused by the transport of oxide ions through the grain boundaries between ZnO grains. It was found that the recovery process is affected by the crystal structure of Bi2O3 particles, which exist at the boundaries between ZnO grains.