A semiconductor device includes a substrate, a first active fin, a second active fin, a dummy fin and a first gate structure. The first and the second active fin are on the substrate and extend along a first direction. The dummy fin is disposed between the first active fin and the second active fin, and extends in the first direction. The dummy fin includes a plurality of layers, and each of the layers includes a material different from another layer. The first gate structure crosses over the dummy fin, the first and the second active fins.