Composite particles consisting of a polymer core and inorganic shell are formed by suspension polymerization. The electrorheological (ER) behavior of the suspensions is studied in relation to surface conditions. When the composite particles are subjected to jet-stream agitation, the ER effects are strikingly enhanced. Although the inorganic shell on the polymer particles is responsible for the ER effect, the impurities which were derived from the polymerization reaction must be removed from the surfaces. The suspensions are nonconducting, even though the polymer cores are coated with conducting particles. In composite particles, the thin layer of polymer prevents direct contact of inorganic particles which are arranged in layers on the polymer core. Since the density of composite particles is comparable to that of oil, the suspensions have long-term stability against sedimentation.