The use of a number of perovskite phases M″ M′O3−x, as the only forming additive in ZnO ceramics, produces a high nonlinearity index, α(up to 45), where M′ is a multivalent transition‐metal ion and M″ is an alkaline earth or a rare‐earth ion. From this study, the formation parameters crucial to high nonlinearity, such as nonstoichiometry in the as‐received ZnO powder, low x values of the additives and fast cooling rate after the sintering, are explainable on the basis of a depletion layer formation at the presintering stage. This is because of the surface states arising out of the chemisorbed oxygen. The depletion layer is retained during sintering as a result of the higher valence state of M′ ions, preferentially present at the grain‐boundary regions. The fast cooling freezes in the high‐temperature concentration of donor‐type defects, thereby decreasing the depletion layer width.