The composite particles with polymeric core particles covered by Fe/sub 3/O/sub 4/ (magnetite) shell were synthesized in order not only to reduce the density of the particle but also to extend its applicability into biological system using magnetic nanoparticles. The magnetorheological (MR) characteristics of the composite suspension were investigated. The MR fluid exhibited the Bingham plastic behavior with a yield stress under an applied magnetic field. Without an applied magnetic field strength, the MR suspension showed a non-Newtonian fluid behavior with a slope of 0.7, compared to a typical Newtonian fluid behavior with a slope of 1.0. The shear thinning behavior of MR fluid under applied magnetic fields showed that the shear viscosity decreases with a shear rate similar to shear thinning of polymer melts or solutions.