The conductive, dielectric, and surface properties of several water-free polymer or inorganic material based ER fluids, as well as their response times, are investigated to elucidate the physical ground of the electrorheological (ER) effect. It is found that the slow polarization, especially the interfacial polarization rather than the Debye polarization, might be responsible for the observed phenomena. A possible ER mechanism is proposed as follows : a large interfacial polarization would facilitate the particle to attain a large amount of charges on the surface, then leading to the turn of particle along the direction of an electric field to form a fibrillation structure; the strength of fibrillation chains is thus determined by the particle polarization force, i.e., the particle dielectric constant. The rationality that the marked interfacial polarization would likely drive the particles to turn is theoretically addressed on the basis of experimental results.