A method for fabricating semiconductor device includes the steps of: providing a substrate having a first region and a second region; forming a first fin-shaped structure on the first region and a second fin-shaped structure on the second region; forming a first spacer adjacent to the first fin-shaped structure and a second spacer adjacent to the second fin-shaped structure; and using the first spacer and the second spacer as mask to remove part of the substrate for forming a third fin-shaped structure on the first region and a fourth fin-shaped structure on the second region, in which the third fin-shaped structure includes a first top portion and a first bottom portion and the fourth fin-shaped structure includes a second top portion and a second bottom portion;