A coupling assembly for a motor-driven power steering system may include a coupling case connected to a motor shaft, a worm shaft including a flange formed at one end portion of the worm shaft; a coupling member disposed between the coupling case and the flange, wherein the coupling member is fitted in the coupling case and is rotatably coupled to the motor shaft formed in the coupling case so that the coupling member integrally rotates with the coupling case, and an elastic protrusion that integrally protrude from lateral side of the coupling member.