A semiconductor device including a substrate, a gate dielectric layer, a gate, a pair of source/drain regions and a stressed layer is disclosed. The gate dielectric layer is disposed on the substrate and the gate whose top area is larger than its bottom area is disposed on the gate dielectric layer. The source/drain regions are disposed in the substrate next to the sidewalls of the gate. The stressed layer is disposed on the substrate to cover the gate and the source/drain regions.