A cord dimmer switch includes a main body, a power cord and a rotating part. The main body includes a circuit device installed in the main body, a through hole formed at the top of the main body, and a rectangular notch formed on a corresponsive adjacent side of the main body. The power cord is connected to both ends of the main body for supplying power to the circuit device and a lamp, and the circuit device has a dimming column. The rotating part is connected to the dimming column to drive the dimming column to rotate synchronously. The rotating part includes a turntable corresponsive to the rectangular notch and a knob corresponsive to the through hole, a part of the periphery of the turntable is protruded out from the rectangular notch, and the top edge of the knob is protruded out from the through hole.