A semiconductor device includes a substrate, a gate dielectric layer on the substrate, and a gate electrode stack on the gate dielectric layer. The gate electrode stack includes a metal filling line,a wetting layer, a metal diffusion blocking layer, and a work function layer. The wetting layer is in contact with a sidewall and a bottom surface of the metal filling line. The metal diffusion blocking layer is in contact with the wetting layer and covers the sidewall and the bottom surface of the metal filling line with the wetting layer therebetween. The work function layer covers the sidewalland the bottom surface of the metal filling line with the wetting layer and the metal diffusion blocking layer therebetween. A method of manufacturing the semiconductor device is also provided.