An adjustable wrench includes a handle and a head is pivotably connected to the handle. The head has a guide hole through which a pin extends and the pin has a side hole defined radially so as to receive a positioning unit therein. A clamping device has a fixed jaw and a movable jaw, the fixed jaw has a travel path and the movable jaw has a toothed rod. A slide slot is defined in one side of the fixed jaw which has a board inserted into the slot of the head. The fixed jaw has an opening in which a push spring, a stop plate and a worm rod are received. The movable jaw is slidably connected to the slide slot. The toothed rod is located in the travel path and engaged with the worm rod.