An impact driver includes an outer sleeve, an inner core, at least one first pin, a moving mechanism, and a shank. The inner core is telescopically and rotatably received in the outer sleeve. The inner core has at least one first helix groove therein. The first pin slidably protrudes from the inner surface of the outer sleeve. The moving mechanism can move the first pin to fit the first helix groove. The shank is connected to the inner core.