A metal gate process for polishing and oxidizing includes the following steps. A first dielectric layer having a trench is formed on a substrate. A barrier layer and a metal layer are formed sequentially to cover the trench and the first dielectric layer. A first chemical mechanical polishing process including a slurry of H2O2 with the concentration of 0˜0.5 weight percent (wt. %) is performed to polish the metal layer until the barrier layer on the first dielectric layer is exposed. A second chemical mechanical polishing process including a slurry of H2O2 with the concentration higher than 1 weight percent (wt. %) is performed to polish the barrier layer as well as oxidize a surface of the metal layer remaining in the trench until the first dielectric layer is exposed, thereby a metal oxide layer being formed on the metal layer.