A method for fabricating semiconductor device is disclosed. The method includes the steps of: providing a substrate having a logic region and high-voltage (HV) region; forming a first gate structure on the logic region and a second gate structure on the HV region; forming an interlayer dielectric (ILD) layer around the first gate structure and the second gate structure; forming a patterned hard mask on the HV region; and transforming the first gate structure into a metal gate.