The influence of additives and secondary phases on the sintering behavior of zinc oxide-based varistors has been investigated in this study. Addition of Bi2O3 and Sb2O3 to ZnO results in rapid shrinkage of the specimens at around 1000 degrees C. This rapid shrinkage is attributed to the reaction between the pyrochlore phase and ZnO. This reaction has been verified by investigating the system of ZnO and a synthesized pyrochlore phase ZnBi1.5Sb1.5O7. This pyrochlore phase has a cubic symmetry, with a lattice parameter a(o) = 10.4438 Angstrom. ZnBi1.5Sb1.5O7 is found to react with ZnO at 1000 degrees C to lead to the formation of Bi2O3, which accelerates the sintering rate of ZnO through a liquid-phase sintering mechanism. On the other hand, the spinel phase alpha-Zn7Sb2O12 hinders the sintering process as well as the grain growth of ZnO.