Abstract: A motorized control for a derailleur for bicycles including a box accommodating a motor. The motor has a drive axle pivotally connected to a worm for actuating a transmission mechanism comprising a drive gear, a couple of driven gears engaging with the drive gear, and a control wheel. The bottom side of the control wheel is provided with two symmetrical posts round which a cable may be wound. Two microswitches are disposed at the outer periphery of the control wheel. When the posts of the control wheel touches the respective microswitches, the rotational angle of the control wheel may be controlled and the control wheel positioned. The box is mounted on the bicycle and electrical switches are installed at the handlebar. When the motor rotates in a clockwise or counter-clockwise direction, the cable may be controlled via the control wheel to become taut or relaxed to thereby control the speed of the bicycle.