The CoO-doping effect on impulse aging behavior of the Zn–Pr–Co–Cr–Dy varistors was investigated in the range of 0.5–2.0 mol%. The clamp voltage ratio (K) increased with increasing CoO content. The varistors doped with 0.5 mol% CoO exhibited the best clamping characteristics, which the K value is 2.05–2.53 at an impulse current of 400–1200 A. The best stability was obtained from the varistor doped with 1.0 mol% CoO, withinline image, %Δα=−4.4%, and %ΔJL=−2.3% after applying the impulse stress of 1200 A.