A semiconductor device includes a semiconductor substrate, a first fin structure and a second fin structure. The first fin structure includes a first fin and at least two first nano wires disposed above the first fin, and the first fin protrudes from the semiconductor substrate. The second fin structure includes a second fin and at least two second nano wires disposed above the second fin, and the second fin protrudes from the semiconductor substrate. Each first nano wire has a first width different from a second width of each second nano wire.