The degradation phenomena caused by dc and ac biasing in non‐Ohmic ZnO ceramics are studied from the viewpoints of voltage (V)‐current (I) characteristics, dielectric properties, and thermally stimulated current (TSC). As a result, it is concluded that the degradation caused by dc biasing is attributed to the asymmetrical deformation of Schottky barriers, due to ion migrations in Bi2O3‐rich intergranular layers and in the depletion layers of the Schottky barriers; and that the degradation caused by ac biasing is attributed to the symmetrical deformation of the Schottky barriers, due to ion migration in the depletion layers of the Schottky barriers. Also, the relationship between the thermal runaway life of non‐Ohmic ZnO ceramics and biasing conditions, such as biasing temperature and bias voltage, is obtained.