An image sensor is provided including a pixel array, a correlated double sampling (CDS) unit, an analog-digital converting (ADC) unit, a control unit, and an overflow power voltage control unit. The pixel array includes at least one unit pixel that generates accumulated charges corresponding to incident light in a photoelectric conversion period and outputs an analog signal based on the accumulated charges in a readout period. The CDS unit generates an image signal by performing a CDS operation on the analog signal. An ADC unit converts the image signal into a digital signal. A control unit controls the pixel array, the CDS unit, and the ADC unit. An overflow power voltage control unit controls an overflow power voltage to have a low voltage level in the photoelectric conversion period and controls the overflow power voltage to have a high voltage level in the readout period.