A semiconductor device comprises a substrate, a semiconductor fin, a first isolation structure and a first dummy structure. The semiconductor fin comprises a first sub-fin and a second sub-fin protruding from a surface of the substrate. The first isolation structure is disposed in the semiconductor fin used for electrically isolating the first sub-fin and the second sub-fin. The first dummy structure is disposed on the first isolation structure and laterally extends beyond the first isolation structure along a long axis of the semiconductor fin, so as to partially overlap a portion of the first sub-fin and a portion of the second sub-fin.