A semiconductor device structure and a method for forming a semiconductor device structure are provided. The semiconductor device structure includes multiple semiconductor nanostructures over a substrate and two epitaxial structures over the substrate. Each of the semiconductor nanostructures is between the epitaxial structures, and the epitaxial structures are p-type doped. The semiconductor device structure also includes a gate stack wrapping around the semiconductor nanostructures. The semiconductor device structure further includes a dielectric stressor structure between the gate stack and the substrate. The epitaxial structures extend exceeding a top surface of the dielectric stressor structure.