This paper presents a surface finishing method based on magnetorheological (MR) fluid to obtain low-loss coplanar waveguides on low resistivity silicon. CPWs with different lateral dimension but having identical 50 Ω characteristic impedance are evaluated. In addition, finite ground effects on CPW performances before and after magnetorheological finishing (MRF) are investigated. In all cases, CPWs treated with the MR fluid-based finishing method reveal much lower attenuation constants compared to original ones owing to reduced conductor roughness. The proposed MRF scheme can be applied to smoothen three dimensional high frequency structures and dramatically improve conductor roughness.