An adjustment mechanism for armrests is revealed. An armrest includes an armrest surface and a lower plate fastened to form a space for receiving a movable block. Thus the movable block is moved forward and backward and positioned in relation to the lower plate. A rectangular slot is disposed on the movable block and a rectangular piece is moved and positioned transversely therein. A round slot is set on the rectangular piece while a hat-shaped disc is mounted in the round slot and fastened elastically on a projecting rod of a bottom frame. By a positioning bump on the hat-shaped disc elastically mounted into or separated from a positioning hole arranged circularly on the projecting rod, the hat-shaped disc and the bottom frame are rotated and positioned in relation to each other in a 360 degree circle. Thereby the armrest can be moved transversely and rotated 360 degrees.