A color image forming system including a photosensitive drum, a charger which charges the photosensitive drum, and a laser scanning unit which is installed below the photosensitive drum and radiates light onto the charged photosensitive drum, to form an electrostatic latent image. The system further includes a plurality of developing units with toner of at least four colors such as yellow, magenta, cyan, and black, arranged at different heights along the outer surface of the photosensitive drum to develop the electrostatic latent image with the toner when a developing roller installed in each of the developing units is maintained at a developing gap with the photosensitive drum. A transfer unit transfers the developed image onto a piece of paper, and a fusing unit is installed above the photosensitive drum and fuses the transferred image onto the piece of paper. The developing units may be arranged in the order of magenta, cyan, yellow, and black.