A method for fabricating semiconductor device includes the steps of: providing a substrate having a first region and a second region; forming a first gate structure on the first region and a second gate structure on the second region; forming a first spacer around the first gate structure; forming a first epitaxial layer adjacent to two sides of the first spacer; forming a buffer layer on the first gate structure; and forming a contact etch stop layer (CESL) on the buffer layer on the first region and the second gate structure on the second region.