In this work, the effect of oxygen (O-2) plasma treatment on methylsilesequiazane (MSZ) dielectric was investigated for CMP process. The low-dielectric-constant (low-kappa) MSZ were prepared by spin-on glass (SOG) process. The resultant wafers were followed by O-2-plasma treatment. After O-2-plasma treatment, the CMP process was implemented. Electrical and material analyses were utilized to explore the characteristics of post-CMP MSZ. Experimental results showed that the polish rate of MSZ film with O-2 plasma pre-treatment was increased as much as twice magnitudes than that of the MSZ without O-2-plasma pre-treatment. In addition, the post-CMP MSZ exhibited superior electrical properties. These results clearly indicated the modification surfaces resulted from O-2-plasma treatment facilitated CMP MSZ, and after CMP polishing, the MSZ film still maintained low-k quality.