A adjustable wrench comprises a body used to operate the adjustable wrench including a fixed jaw extending outward therefrom and having a holder mounted on a top end thereof, including an arcuate recess fixed on a middle portion of a front side thereof; a movable jaw pivoted in the holder, serving as engaging a workpiece, and including a guiding rib relative to the arcuate recess extending from a bottom end thereof to be adjusted and moved along the arcuate recess; a locking mechanism axially displacing in the cavity of the body and including a control unit to operate the locking mechanism and to drive a pushing member, the control unit including locking and unlocking positions so as to move the pushing member across the notch of the arcuate recess and then to bias against the guiding rib, thereafter, the pushing member is driven to move downward, thus disengaging the guiding rib to slide freely.