Abstract-A core/ shell particle with a poly(methyl methacrylate) (PMMA) core and a conducting polyaniline (PANI) shell was prepared and used as a dispersed phase for an electrorheological (ER) uid, which is a suspension of polarizable particles in a non-conducting medium. PMMA particles, which were obtained using the dispersion polymerization method and spherical in shape with a monodisperse size distribution, were used as the core material for three different particle sizes. A PANI shell was introduced to the PMMA particle surface through in situ polymerization of aniline by an oxidant within an aqueous acidic solution, and its thickness was controlled via the amount of aniline monomer. These synthesized particles were dispersed in a silicone oil to prepare an ER uid. The ER uid, possessing Newtonian characteristics in the absence of an external electric eld, exhibited a viscoelastic solid behavior with a non-vanishing yield stress, which were systematically controlled by the applied electric eld strengths.