The invention discloses a semiconductor device and a manufacturing method thereof. The method includes the steps that a semiconductor substrate is provided; an interlayer dielectric layer is formed on the semiconductor substrate, and a groove exposing the semiconductor substrate is formed in the interlayer dielectric layer; a gate dielectric layer is formed at the bottom of the groove; a tungsten layer is formed on the gate dielectric layer and the interlayer dielectric layer, and the groove is filled with the tungsten layer; a protection layer is formed on the tungsten layer; chemical mechanical polishing is conducted to remove the part, outside the protection layer and the groove, of the tungsten layer, so that a gate metal layer is formed. According to the manufacturing method of the semiconductor device, the defect that the finally-formed gate metal layer fractures is overcome, and a smooth surface is obtained.