The effect of Nb2O5 addition on impulse aging behavior of ZnO-V2O5-MnO2-Co3O4-Dy2O3-based varistors was investigated. The varistors doped with 0.25 mol% Nb2O5 exhibited the best clamp characteristics, in which the clamp voltage ratio was in the range of K = 1.65-2.50 at an impulse current of 1-50 A. The varistors doped with 0.1 mol% Nb2O5 exhibited the best electrical stability by marking %Delta E-1 (mA) = -1.0%, %Delta alpha = -2.8%, and %Delta J(L) = 31.5% after applying the impulse current of 100 A.