A method for fabricating semiconductor device is disclosed. The method includes the steps of: providing a substrate having at least a gate structure thereon and an interlayer dielectric (ILD) layer around the gate structure; forming a hard mask on the gate structure and the ILD layer; forming a first patterned mask layer on the hard mask; using the first patterned mask layer to remove part of the hard mask for forming a patterned hard mask; and utilizing a gas to strip the first patterned mask layer while forming a protective layer on the patterned hard mask, wherein the gas is selected from the group consisting of N2 and O2.