A quick adjustable clamp includes a first clamp jaw, a second clamp jaw, a driving screw and a nut. The second clamp jaw is extended with a receiving section. The receiving section is formed with a notch facing the first clamp jaw. The driving screw has one end pivotally connected to the first clamp jaw. The nut is mounted around the driving screw and configured to be movably received in the notch. For quick adjusting a span of the clamp, the nut can be removed from the notch, quick moved along the driving screw, and then reengaged by the notch.