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 plurality of fin-shaped structures on the substrate; forming a gate layer on the fin-shaped structures; forming a material layer on the gate layer; patterning the material layer for forming sacrificial mandrels on the gate layer in the first region; forming sidewall spacers adjacent to the sacrificial mandrels; removing the sacrificial mandrels; forming a patterned mask on the second region; and utilizing the patterned mask and the sidewall spacers to remove part of the gate layer.