A semiconductor device includes: a substrate having a first region and a second region; a first fin-shaped structure on the first region and a second fin-shaped structure on the second region; a shallow trench isolation (STI) around the first fin-shaped structure and the second fin-shaped structure; a first oxide layer on the first fin-shaped structure; a second oxide layer on and directly contacting the first oxide layer and the STI; and a third oxide layer on the second fin-shaped structure, wherein a thickness of the third oxide layer is less than a thickness of the first oxide layer.