Magnetorheological (MR) fluid, which is composed of soft magnetic particles with little hysteresis dispersed in a non-magnetic liquid medium, is a phase controllable material under an external magnetic field. Magnetic microspheres consisting of a polyamide 6 (PA6) shell wrapping a magnetic carbonyl iron (CI) particle as a core were prepared using an in-situ polymerization method and adopted as a MR material to enhance the dispersion stability and mechanical properties. The physical characteristics of PA6 magnetic microspheres, such as morphology, magnetic properties, components of magnetic microspheres, and rheological properties, were investigated. The MR characteristics were measured using rotational and oscillatory tests. The sedimentation rate of the pure CI and PA6/CI suspension were also investigated.