A semiconductor device includes a semiconductor substrate having a first region and a second region, insulators, gate stacks, and first and second S/Ds. The first and second regions respectively includes at least one first semiconductor fin and at least one second semiconductor fin. A width of a middle portion of the first semiconductor fin is equal to widths of end portions of the first semiconductor fin. A width of a middle portion of the second semiconductor fin is smaller than widths of end portions of the second semiconductor fin. The insulators are disposed on the semiconductor substrate. The first and second semiconductor fins are sandwiched by the insulators. The gate stacks are over a portion of the first semiconductor fin and a portion of the second semiconductor fin. The first and second S/Ds respectively covers another portion of the first semiconductor fin and another portion of the second semiconductor fin.