An automatic voltage control circuit controls a power supply unit to adjust a supply voltage provided by the power supply unit. The automatic voltage control circuit includes an oscillating unit, a frequency-comparing unit, and a control unit. The oscillating unit generates an oscillating signal. The frequency-comparing unit compares the oscillating frequency of the oscillating signal with at least one predetermined threshold frequency. The control unit controls the power supply unit to adjust the supply voltage according to the comparing result generated by the frequency-comparing unit.