In this work, chemical-mechanical polishing (CMP) of the organic polymer, methylsilsesquioxane (MSQ), has been investigated. For conventional silicate-based slurry, the CMP removal rate of MSQ is low and many scratches are formed at the surface. Moreover, the dielectric properties of a post-CMP MSQ film are degraded in comparison to the as-cured MSQ. We have proposed a reliable process for the CMP of MSQ which includes a slurry of additive and a post-CMP NH3 plasma treatment. Experimental results show that the modified slurry provides a high polishing rate and uniform surface topography. In addition, the NH3 plasma process can form a thin nitrogen-containing layer on the post-CMP MSQ surface, which enhances the resistance to moisture absorption and copper diffusion.