A semiconductor device has a substrate, a first dielectric fin, and an isolation structure. The substrate has a first semiconductor fin. The first dielectric fin is disposed over the substrate and in contact with a first sidewall of the first semiconductor fin, in which a width of the first semiconductor fin is substantially equal to a width of the first dielectric fin. The isolation structure is in contact with the first semiconductor fin and the first dielectric fin, in which a top surface of the isolation structure is in a position lower than a top surface of the first semiconductor fin and a top surface of the first dielectric fin.