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 electrorheolgoical effect, It is found that the slow polarization, especially the interfacial polarization rather than the Debye polarization, should be responsible for the observed phenomena. Combining with the recent achievements (both theoretical and experimental) in the electrorheological field we argue that it is the interfacial polarization that does play a central role in electrorheological response; The particle re-orientation process is actually resulted from the large interfacial polarization.