Nowadays the most important part of varistors is made from doped ZnO. The typical varistor microstructure consists in a large number of n–p–n junctions. ZnO grains are intrinsic n-type semiconductors because of the displacement of zinc atoms in interstitial positions and the formation of oxygen vacancies. The addition of some dopants (for instance bismuth and antimony oxides) allows creating a p-type semiconduction at the grain boundaries. In our study, ZnO-based varistor with standard composition was sintered by microwave and in a conventional furnace with the same sintering temperatures and dwell times. Electrical characterizations after direct microwave sintering showed that these samples presented a high electrical conductivity which avoids getting a good current–voltage non-linearity. This high conductivity could be due to higher concentrations in interstitial zinc and oxygen vacancies after the microwave process. It is assumed that microwaves cause a displacement of the equilibria of those reactions leading to a partial reduction of the samples. A post thermal treatment in a conventional furnace at 650 °C for 24 h under oxygen atmosphere was realized so as to reach the thermodynamic equilibrium. After this treatment the electrical conductivity drastically decreased supporting the idea that the defects concentrations have also decreased.