Abstract: An apparatus and a method for compensating the gray-scale values of an image scanned by a contact image sensor. The gray-scale compensating apparatus includes: a power supply for providing a fixed level of voltage source, a contact image sensor for scanning an image and outputting image voltages VIMG of pixels of the image, or scanning a white pixel and outputting the white-level voltage VP of the white pixel, an analog/digital converter, to receive the fixed voltage from the power supply and the image voltage VIMG or the white-level voltage VP, a microprocessor to calculate the average of n white-level values G(x, y), where n is an integer, and the average value G′(x)=(G(x, 0)+G(x, 1)+ . . . +G(x, n−1))/n. The microprocessor also determines the gray-scale values g′(x, y) of the image being compensated by calculating the gray-scale values g(x, y) of the image not compensated in accordance with the value of G′(x).