A method for fabricating semiconductor device is disclosed. The method includes the steps of: providing a substrate having a first region and a second region defined thereon; forming a material layer on the substrate; forming a plurality of first mandrels on the material layer of the first region and the second region; forming first spacers adjacent to the first mandrels; forming a hard mask on the first region; trimming the first spacers on the second region; removing the first mandrels; using the first spacers to remove part of the material layer for forming a plurality of second mandrels; forming second spacers adjacent to the second mandrels; removing the second mandrels; and using the second spacers to remove part of the substrate for forming a plurality of fin-shaped structures.