A method of forming a semiconductor device is disclosed. At least one suspended first semiconductor nanowire and two first semiconductor blocks at two ends of the first semiconductor nanowire are formed in a first area, and at least one suspended second semiconductor nanowire and two second semiconductor blocks at two ends of the second semiconductor nanowire are formed in a second area. A transforming process is performed, so the first semiconductor nanowire is transformed into a nanowire with stress, and the second semiconductor blocks are simultaneously transformed into two blocks with stress. First and second gate dielectric layers are formed respectively on surfaces of the nanowire with stress and the second semiconductor nanowire. First and second gates are formed respectively across the nanowire with stress and the second semiconductor nanowire.