Combining the improved effective-mediume theory (EMT) with the feature of sensitive semiconductor ceramics, we proposed a simple and quantitatively physical model that can be used to describe the relation between micros tructure feature and proprety of sensitive semiconductor ceramics. We used this model to describe the electrical propreties of nonlinear ZnO ceramics by, and found that the results of the model are consistent with experimental results and the results obtained by the quentum theory of semiconductor conduction.