Abstract: An adjusting mechanism for adjusting arm rest of a chair includes an inner tube with an axial groove in an outside thereof and multiple positioning holes are located within the axial groove. An outer tube is mounted to the inner tube and an arm rest is connected to a top end of the outer tube. An engaging portion protrudes inward from an inside of the outer tube and movably engaged with the axial groove of the inner tube, so that the outer tube can be moved relative to the inner tube. An operation member is pivotably connected to the outer tube and includes a hook which extends through the outer tube and is engaged with one of the positioning holes. A guide plate is located above the hook and movably accommodated in a guide slot in the outer tube so that the operation member is operated in stable status.