Abstract: Magnetorheological fluids were prepared by using carbonyl iron as magnetic particles and SiO₂ nanoparticles with different specific surface areas (150, 200, 380 m²·g^-1) as thixotropic agent. The effects of SiO₂ with different specific suface areas on the rheological properties and sedimentation stability of magnetorheological fluids were studied. The results show that the larger the specific surface area of SiO₂, the greater the saturation magnetization and the relative permeability of the magnetorheological fluid. With the increase of the specific surface area of SiO₂, the off-state viscosity of the magnetorheological fluid increased rapidly and then decreased slightly, and the the shear stress, yield stress and sedimentation stability increased. When the specific surface area of the SiO₂ was 380 m²·g^-1, the magnetorheological fluid showed an excellent controllability.