Based on microstrueture image of typical nonliear ZnO ceramics (ZnO varistors), an intuitive and simple physical model of conduction has been proposed, that can quantitatively discribe the characteristics of ZaO varistors. According to this mode], in low bias voltage, electrons pass through SIS heterojunction and SS homojuction (in parallel) mainly by thernionic emission. Above breakdown voltage, SS homojuction is active juction of conduction. In this case, the reverse-biased barrier layer is transformed from depletion layer to strong inversion layer. The thickness of barrier top becomes thinner, and tunnelling electron flux rises sharply. Hence the nonliear conduction is produced. In high bias voltage, conduction is limited by grain resistivity.