A semiconductor device includes a semiconductor fin protruding from a substrate, a gate electrode over the semiconductor fin, a gate insulating layer between the semiconductor fin and the gate electrode, source and drain regions disposed on opposite sides of the semiconductor fin, a first stressor formed in a region between the source and drain regions. The first stressor is a grading strained stressor including multiple graded portions formed at graded depths. The first stressor is configured to create one of a graded compressive stress or a graded tensile stress.