An open end wrench (10) includes first and second jaws (23, 24) formed on a jaw portion (22). The first jaw (23) includes an arcuate sliding groove (27) having two support walls (272, 273) and an arcuate sliding wall (271) between the support walls (272, 273). A slide (30) is received in the sliding groove (27) and includes an arcuate sliding face (31) slideable along the sliding wall (272). An arcuate guiding slot (35) is formed in the slide (30). A guide (28) is fixed in the sliding groove (27) and received in the guiding slot (35). The guiding slot (35) includes a pressing end (352). An elastic device (40) has two ends respectively abutting the guide (28) and the pressing end (352) of the guiding slot (35) for biasing the slide (30) to a natural position. The slide (30) includes a first wrenching face (32) having two wings (36). A spacing (L) is formed between each wings (36) and one of the support walls (272, 273).