The organic silsesquioxane, methylsilsesquioxane (MSQ), has a low dielectric constant because of its lower film density compared to thermal oxide. However. the quality of MSQ film is degraded by the damage of oxygen plasma and hygroscopic behavior during photoresist stripping. In this work, we studied the N2O plasma treatment for improving the quality of MSQ. The leakage current of MSQ decreases as the N2O plasma treatment time is increased. The dielectric constant of N2O plasma-treated sample remains constant (similar to 2.7). In addition, the thermal stability of MSQ film can be enhanced. The role of N2O plasma is to convert the surface layer of organic MSQ into inorganic type by decomposition of the alkyl group and thus form a passivation layer. The inert passivation layer enhances the resistance to moisture uptake and O-2 plasma attack. Therefore, N2O plasma-treatment greatly improves the quality of low k MSQ film and removes the issue of photoresist stripping in the integrated process.