The varistor characteristics of 0.1-3.0 at. % Co-doped ZnO have been investigated. Three kinds of deep bulk trap levels of 0.16, 0.25, and ~0.31 eV were identified as donor levels by admittance spectroscopy. Oxygen vacancy ( V o ·) appeared preferentially in 0.5-3.0 at. % Co-doped ZnO. From J-E characteristics nonohmic behavior was seen in this binary system while the nonlinear coefficient α changed between 3 and 35 depending on the composition. For 1.0 at. % Co-doped ZnO, two distinguishable activation energies of 0.65 eV and 1.04 eV related with grain boundary phenomena were confirmed above and below ~520 K by impedance and modulus spectroscopy. It is believed that the varistor behavior of Co-doped ZnO stems from the formation of double Schottky barrier by the valence change of Co ions in ZnO grains as well as oxygen chemisorption at the grain boundaries on heating and subsequent cooling.