An open end wrench includes first and second jaws. The first jaw includes an arcuate sliding groove having two support walls and an arcuate sliding wall between the support walls. A slide is slideably received in the sliding groove and includes an arcuate sliding face slideable along the sliding wall. Two wings respectively extend from top and bottom faces of the slide. Top and bottom extension faces extend from a wrenching face of the slide and are located on inner faces of the wings. When the open end wrench drives a workpiece to rotate, an outer face of each wing is partially in contact with the second jaw. Force imparted from the workpiece to the wings is transmitted to the second jaw.