The electrical and dielectric properties and the microstructures of a polynary ZnO-based varistor ceramics with 14000 times impulse current aging test are measured. The relationship between defect structure and impulse current aging is mainly investigated. It is found that the electrical properties decrease rapidly with impulse aging and the dimensional effect of ZnO varistor ceramics is dominated not only by grain but also by grain boundary. Additionally, four defect relaxations are found at different temperatures by using dielectric spectra. Two defect relaxations appearing below -60 ℃ with activation energies about 0.24 eV and 0.35 eV are identified to be intrinsic defects originating from interstitial Zn L(Zni··) and vacancy oxygen L(VO·), which are not affected by impulse current aging. Other two relaxations appearing above 80 °C are suggested to be extrinsic defects originating from trap levels L(ingr) at intergranular phase and trap levels L(gb) at grain-boundary interfaces, respectively. Only L(gb) decreases from 0.84 eV to 0.76 eV due to impulse current aging while other trap levels keep unchanged. It is further proposed that L(gb) is responsible mainly for the electrical property and stability of ZnO ceramics.