The invention provides a semiconductor device structure and a forming method thereof. The semiconductor device structure comprises a substrate layer, a silicon carbon layer, a carbon base material layer, a gate stacker, a source, a drain and a metal silicide layer, wherein the silicon carbon layer is formed on the substrate layer; the carbon base material layer is formed on the silicon carbon layer; the gate stacker is formed on the carbon base material layer; the source and the drain are formed in the carbon base material layer; and the metal silicide layer is formed on the source and the drain. In the invention, the carbon base material layer, such as graphene, is used as a channel layer, and therefore, the speed of the device is greatly increased.