The spherical, monodisperse polymer microshperes (PAPMMAs) consisting of a poly(methyl methacrylate) (PMMA) core and a polyaniline (PANI) shell were prepared and adopted as dispersed materials for electrorheological (ER) fluids, in which electrical conductivity was originated primarily from the surface-coated conducting PANI layer. These surfaces were rough and irregular. Three different core sizes of PAPMMAs (2, 4.5, and 9 mum) with the same weight ratio of PANI, the larger core having the thicker PANI shell, were prepared. PAPMMAs that have similar shell thicknesses but different core sizes were also prepared in order to examine the particle size effect on ER performance. Performances of PAPMMA-based ER fluids were enhanced with increasing core particle size. The same result was obtained for the PAPMMA systems having the same shell thickness. Dielectric spectra of these ER fluids were found to provide additional information on both analyzing their electrical polarization properties and interpreting the flow behavior of the PAPMMA-based ER fluids.