The Wagner theory, which describes the interfacial polarization in heterogeneous systems, was employed to model the electrorheological(ER) effect under the presumption that the shear stress increment is induced by the interfacial polarization. The currently observed experimental facts, such as the yield stress of some ER fluids, decreases with the applied field frequency increasing or the environment temperature decreasing, while that of other fluids increases with the frequency decreasing or temperature increasing; the strongest ER effect is usually observed in the suspension with the dispersed particle conductivity around 10 ุ7 S/m; the particle dielectric loss tangent of a good ER fluid usually is above 0.10 at 1000 Hz; and the fluid with a high conductive particle usually has a short response time, can be satisfactorily understood with the extended Wagner model. The Wagner-polarization-induced maximum yield stress of a heterogeneous-type ER fluid is estimated around 7 kPa under the presumption that the dielectric constants of the solid particle and the liquid medium are 10 and 2, respectively, the particle volume fraction is 35%, and the applied electric field strength is 3 kV/mm. It is concluded that the ER effect may substantially correlate with the Wagner polarization, which would help in understanding the mechanism of the ER effect and would provide a strategy for designing high performance ER fluids.