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, with %Delta E1mA/cm2 = -0.7%, %Delta alpha = -4.4%, and %Delta J(L)=-2.3% after applying the impulse stress of 1200 A.