An image sensor is provided. The image sensor includes a pixel array, an analog-to-digital converter, and a processor. The analog-to-digital converter converts a black level reference signal and a pix signal from the pixel array into a first digital signal and a second digital signal, respectively. The processor obtains a black level reference value according to the first digital signal, and obtains a compensation coefficient according to the black level reference value, a maximum digital level of the analog-to-digital converter and a full signal range value. The processor obtains pix data according to the compensation coefficient, the black level reference value and the second digital signal.