To study the rheological properties and polishing performance of magnetorheological fluids (MRFs) prepared with SiO 2@ carbonyl iron (CI) core-shell particles, SiO 2@ CI core-shell particles with different shell thicknesses were synthesized by an improved Stöber method controlling the amount of tetraethyl orthosilicate (TEOS). Then, four types of MRFs which dispersed with core-shell particles with thin shells, core-shell particles with thick shells, SiO 2 particles, or carbonyl iron were prepared for sapphire polishing. The results show that the rheological properties of MRF based on thicker shell SiO 2@ CI are enhanced compared to those of MRFs prepared by dispersing abrasive and CI. Comparing with other MRFs, sapphire can obtain better surface quality when using thicker shell SiO 2@ CI. The MRF fabricated by SiO 2@ CI core-shell particles can increase the contact area between the abrasive and the workpiece and enhance the contact stress, which improves the solid-phase reaction rate and mechanical removal efficiency of silica and sapphire, thereby improve the polishing efficiency.