An open end wrench includes first and second jaws. 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. 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. A spring seat is mounted in the pressing end of the guiding slot. The spring seat includes a face pressing against the pressing end. An elastic element has two ends respectively abutting the guide and the other face of the spring seat for biasing the slide to the initial position.