The effects of pre-synthesized V2O5/Sb2O3 compound, SbVO4 and Sb2O3 on the microstructure and electrical properties of ZnO-V2O5 based varistor ceramics have been studied. The general homogeneous microstructure and phase composition of the ceramics have not been influenced by the antimony doping form changes, except for the gradual decrease in average grain size and the increase in spinel formation within the ceramics. The ultimate cause of micro-structural variations with the antimony doping form changes seems to have been the gradual increase in Sb3+ concentration within the ceramics. This change in micro-composition, together with the changes in microstructure, caused the electrical characteristics of the ceramics to vary. The pre-synthesized V2O5/Sb2O3 compound is the most effective antimony dopant in term of its effect on the ceramics.