Abstract: Nanoparticles gelatin-iron oxides composite particles have been prepared with the microemulsion method.The investigations by XRD,SEM, TEM and IR show the ultrafine particles are spheroids coated by gelatin.Their average sizes are 1.5μm~3μm, and particles sizes are 20 nm,about 35~90 iron-oxides particles on each spheroid.The specific saturation magnetization σ_s=62Am²/kg,the coercivity Hc=187Oe and residual magnetism r=10.18 Am²/kg for the composite particles. The coercivity and residual magnetism are higher, and it is shown that the complex supermicropower has hard magnetic properties. Magneticrheological (MR) effects of the MR fluid prepared from the ultrafine particles are superior in lower magnetic field intensity and the subsidence stability of the MRF is excellent compared with carbonyl iron powder.