A semiconductor device including a transistor and a method of forming thereof are provided. The semiconductor device comprises a metal gate electrode. A lower portion of the metal gate electrode fills a channel trench formed at a predetermined region of a substrate, and an upper portion of the metal gate electrode protrudes on the substrate. A gate insulating layer is interposed between inner sidewalls and a bottom surface of the channel trench, and the metal gate electrode. Source/drain regions are formed at the substrate in both sides of the metal gate electrode.