An image processing method and an image forming apparatus using the same. The method of processing includes dividing an image into a predetermined number of blocks, calculating an average of color signals of pixels existing in each of the blocks divided, classifying the blocks into color blocks and monochrome blocks using the calculated average, and determining whether the image is a color image or a monochrome image using a proportion of the color blocks. Accordingly, for a document wherein color and monochrome images are mixed, it is automatically determined whether an image is a color image or a monochrome image.