The relationship between the dielectric dissipation factor D(tan delta) and frequency in the varistors of ZnO-Sb(2)O(3)-BaO and ZnO-Bi(2)O(3)-Sb(2)O(3)-BaO system ceramics has been investigated. A new dielectric dissipation peak was found near 1.5 MHz at room temperature in the ZnO-Sb(2)O(3)-BaO system, and the corresponding electron trapping level is about 0.18 eV. The dissipation peak was considered to result from the intrinsic defect Zn(i)(-). The additive Bi(2)O(3) may play a suppressing effect on the formation of Zn(i)(-).. This assumption was consistent with the experimental fact that degradation phenomenon of varistor ceramics under long-term load voltage is improved by the increasing amount of Bi(2)O(3) additive. (C) 2011 Elsevier Ltd. All rights reserved.