A semiconductor device includes a substrate, a dielectric layer, a first tungsten layer, an interface layer and a second tungsten layer. The dielectric layer is disposed on the substrate and has a first opening and a second opening larger than the first opening. The first tungsten layer is filled in the first opening and is disposed in the second opening. The second tungsten layer is disposed on the first tungsten layer in the second opening, wherein the second tungsten layer has a grain size gradually increased from a bottom surface to a top surface. The interface layer is disposed between the first tungsten layer and the second tungsten layer, wherein the interface layer comprises a nitrogen containing layer. The present invention further includes a method of forming a semiconductor device.