A greatly enhanced breakdown electric field of ∼8 kV/cm was achieved in multi-doping ZnO ceramics, and the role of defects in determining the electrical properties were systematically investigated in this work. At low temperature of around 203 K, it is found that the dielectric loss is composed of dc conduction and two defect relaxation peaks with activation energy at 0.24 eV and 0.37 eV, which can be effectively described according to Debye relaxation theory. At high temperature of 473 K, anther two defect relaxation peaks with activation energy at 0.65 eV and 0.98 eV are found to obey Cole-Davidson function, which are greatly affected by additives and closely related to the electrical properties of ZnO ceramics. Additionally, impedance analysis shows that the grain boundary resistance of ZnO ceramics is increased from 0.56 MΩ to 15.7 MΩ at 473 K and the corresponding activation energy of grain boundary is elevated from 0.23 eV to 1.03 eV. The frequency dependence of the conductivity is interpreted with the Jonscher's law, which indicates that the contribution of dc conduction at low frequency can be evidently suppressed by additives. An equivalent circuit model is demonstrated for expounding the association between enhanced electrical properties and defect relaxation in ZnO ceramics.