A dimming system comprises a controller and an AC-to-DC converter. The controller outputs a control signal in response to a dimming signal. The controller comprises a frequency adjustment circuit and a duty cycle adjustment circuit. The frequency adjustment circuit adjusts the frequency of the control signal in response to the dimming signal. The duty cycle adjustment circuit adjusts the duty cycle of the control signal in response to the dimming signal. The AC-to-DC converter converts an external power and to output a DC signal in response to the control signal so as to drive a light source by the DC signal. When the controller is operated at a first mode, the control signal has a first frequency and a first duty cycle. When the controller switches to a second mode, the control signal has a second frequency and a second duty cycle.