The degradation of non-Ohmic electrical characteristics of ZnO-glass chip varistors due to high-intensity impulse currents was correlated with the deep trap levels investigated by means of deep level transient spectroscopy. Three electron traps located at 0.11, 0.27, and 0.94 eV below the conduction band were observed for chip varistors before current impulse testing. These trap energy depths of the chip varistors after current impulse testing were found to be almost unchanged, but their trap densities and capture cross sections both decreased. The concentration of the trap at 0.94 eV was decreased and ascribed to oxygen vacancies existing at the grain boundaries of the varistor. The electrical degradation phenomenon of the chip varistors is closely related to the reaction between the trap at 0.94 eV and adsorbed oxygen ions at the grain boundaries.