A semiconductor device structure is provided. The semiconductor device structure includes a substrate. The semiconductor device structure includes a first fin structure and a second fin structure disposed over the substrate. The semiconductor device structure includes a first gate stack disposed over the substrate and overlapping the first fin structure. The first gate stack includes a first work function layer, a first gate electrode, and a first hard mask layer, the first gate electrode is over the first work function layer, the first hard mask layer is over the first gate electrode, the first gate electrode has a first convex top surface protruding beyond a first top surface of the first work function layer. The semiconductor device structure includes a second gate stack disposed over the substrate and overlapping the second fin structure.