ZnO varistors with different amount NiO dopant are subjected to the polarity reversal DC stress to examine their degradation properties. After doping NiO, the dependency of the power loss of ZnO varistors on degradation time shows a dropping trend and the J-E characteristics in two axial directions deteriorate alternatively, before and after the reversal. With the aid of the dielectric and complex impedance spectroscopies, two processes during degradation are characterized, namely electrons filling the empty interface states and the migrations of metastable ions that are hypothesized as Zni and Oad. The processes modify the defect structure of the double Schottky barriers at the grain boundaries. As a result, the reverse-biased barrier height gradually increases while the forward-biased barrier decreases, which accounts for the lowering of the power loss and the alternative changes of the J-E characteristics. Due to NiO with the effect of inhibiting the density of Zni and blocking the migration path of Oad, ZnO varistors show enhanced stability against the polarity reversal DC stress.