An expandable armrest of a wheel chair includes a fixed part having a first pivotal portion which includes a first pivotal hole. A movable part is pivotably connected to the first pivotal portion of the fixed part by its second pivotal portion. A first transverse hole and a second transverse hole are defined in the second pivotal portion. A slide member is slidably inserted into the slots of the second pivotal portion and has an inclined hole in which an engaging member is engaged. The engaging member has a first end and a second end. A trace that the engaging member moves in the inclined hole constantly intersects a trace that the first end moves in the first transverse hole and a trace that the second end moves in the second transverse hole, such that the movable part is positioned by controlling the movement of the slide member.