An image processing apparatus is disclosed. The image processing apparatus comprises a dividing module, a calculating module, a judging module and an adjusting module. The dividing module is used for dividing an image into a plurality of sub-images. The calculating module is used for generating a lightness threshold of the image. The judging module is used for comparing an original average lightness of a target sub-image among the plurality of sub-images with the lightness threshold. The adjusting selectively increases the original average lightness of the target sub-image according to the first judging module.