A method for fabricating a semiconductor device includes the steps of: providing a substrate having a NMOS region and a PMOS region; forming a first gate structure on the NMOS region and a second gate structure on the PMOS region; forming a seal layer on the first gate structure and the second gate structure; forming a first lightly doped drain (LDD) adjacent to the first gate structure; forming a second LDD adjacent to the second gate structure; and performing a soak anneal process to boost an oxygen concentration of the seal layer for reaching a saturation level.