A semiconductor device structure is provided. The semiconductor device structure includes an isolation layer formed over a substrate, and a plurality of nanostructures formed over the isolation layer. The semiconductor device structure includes a gate structure wrapped around the nanostructures, and an S/D structure wrapped around the nanostructures. The semiconductor device structure includes a first oxide layer between the substrate and the S/D structure. The first oxide layer and the isolation layer are made of different materials. The first oxide layer is in direct contact with the isolation layer.