A semiconductor device structure is provided. The semiconductor device structure includes a substrate having a fin structure protruding therefrom, an insulating layer is over the substrate to cover the fin structure, a gate structure in the insulating layer and over the fin structure, and source and drain features covered by the insulating layer and over the fin structure on opposing sidewall surfaces of the gate structure. The gate structure includes a gate electrode layer, a conductive sealing layer covering the gate electrode layer, and a gate dielectric layer between the fin structure and the gate electrode layer and surrounding the gate electrode layer and the conductive sealing layer. The gate electrode layer has a material removal rate that is higher than the material removal rate of the conductive sealing layer in a chemical mechanical polishing process.