The effects of amount of Sb added to Bi-Mn-Co-Sb-Si-Cr-Ni-Zr-added ZnO varistors on the tolerance characteristics of electrical degradation were investigated. Furthermore, the effects of addition of both Zr and Y to Bi-Mn-Co-Sb-Si-Cr-Ni-added ZnO varistors (the same composition as one of commercial varistors) on the varistor voltage and leakage current were investigated.ZnO varistors were fabricated by adding Bi 2 O 3 , MnO 2 , Co 3 O 4 , Cr 2 O 3 , NiO, SiO 2 as a basic additives, Y 2 O 3 of 0 -1.0 mol% (Y of 0 -2.0 mol%), or Zr of 0 -0.6 mol% and Sb 2 O 3 of 0 -2.5 mol%, as extra additives to ZnO. Pressed powders were calcined at 600ºC for 5 h in air and sintered at 1150ºC for 3 h in air. Aluminum electrodes were applied to two end faces to measure the voltage-current (V-I) characteristics. The electrical degradation was carried out by applying a dc current of 100 mA/cm 2 in N 2 atmosphere for 30 min. The nonlinearity index α and the standup voltage of V-I characteristic were estimated. The composition of elements in a sample was analyzed by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The crystal structure was analyzed by X-ray diffraction (XRD).Values of nonlinearlity index a bef and a deg before and after the electrical degradation, respectivly, are shown in Fig. 1. It was found that addition of Sb enhances the tolerance characteristics to the electrical degradation and the amount of Sb 2 O 3 addition of approximately 1 mol% of is the optimal. When the amount of added Sb increased, the amount of existence of a Zn 2.33 Sb 0.67 O 4 -type spinel particle evaluated from XRD as shown in Fig. 2 increased, because the amount of existence of Zr compound increased similarly. It was suggested that Zr compound was also related to the electrical degradation characteristic.Varistor voltage decreases a little when the amount of added Zr increases in simultaneous addition of Zr and Y. On the other hand, the value of nonlinearlity index hardly changed. Without Zr, the increase in the leakage current accompanying the increase in the amount of added Y, which has been reported conventionally, was not found in this study as shown in Fig. 3. However, adding Zr of approximately 0.2 -0.45 mol%, the leakage current increased by adding Y of more than 1 mol% and adding Zr of approximately 0.6 mol%, it decreased.