Abstract A ZnO varistor with a breakdown electric field of 180 Vmm-1 in air ambient has been fabricated by cold-pressing of pure ZnO in form of thick film porous semiconductor. The behavior of the varistor at various temperatures was investigated and its non-linear I-V characteristics were recorded. It was shown that the breakdown voltage shifts to a lower electric field with rise of its operating temperature. The higher temperatures causes to higher changes in the breakdown voltage and lower electric field. Also it revealed that the breakdown voltage of the varistor depends on the number of grain boundaries located between two electrodes of the varistor. The breakdown voltage of the varistor can also be affected by the pollutant gases in environment.