An open end wrench includes first and second jaws formed on a jaw portion. The first jaw includes an arcuate sliding groove having an arcuate sliding wall. A slide is received in the sliding groove and includes an arcuate sliding face slideable along the sliding wall. An arcuate guiding slot is formed in the slide. The sliding face, sliding wall and guiding slot are free of holes, grooves, and recesses. A guide is fixed in the sliding groove and received in the guiding slot. The guiding slot includes an abutting end and a pressing end. The abutting end is in contact with the guide when the slide is in an initial position not engaged with a workpiece. An elastic device has two ends respectively abutting the guide and the pressing end of the guiding slot for biasing the slide to the initial position.