A method for fabricating semiconductor device is disclosed. The method includes the steps of: providing a substrate having at least one metal gate thereon, a source/drain region adjacent to two sides of the at least one metal gate, and an interlayer dielectric (ILD) layer around the at least one metal gate; forming a plurality of contact holes in the ILD layer to expose the source/drain region; forming a first metal layer in the contact holes; performing a first thermal treatment process; and performing a second thermal treatment process.