An open end wrench includes a body having a jaw portion for holding a workpiece. The jaw portion includes an arcuate sliding groove, an expanding groove, and a slider movable relative to the arcuate sliding groove along an arcuate path. The jaw includes a first jaw having a force-applying face and an evasive portion adjacent to the force-applying face. When the jaw portion deforms elastically while wrenching the workpiece, the expanding groove increases the expanding effect within an elastic limit of the jaw portion, such that a corner between the force-receiving face in a first rotating direction and a sixth force-receiving face in a second rotating direction can move across the force-applying face into the evasive portion, avoiding damage to the slider and the first jaw resulting from failure in withstanding the reactive force from the workpiece. Thus, the open end wrench has high toughness and a long service life.