Disclosed herein are an image forming apparatus and method that determines whether a photo sensor used for color registration is contaminated and a contamination level thereof so as to automatically clean the photo sensor. The image forming apparatus includes an intermediate transfer medium onto which images having different colors are to be overlapped and transferred, a photo sensor which emits light to receive light reflected by the intermediate transfer medium, a cleaning unit which cleans the photo sensor, and a control unit which controls the cleaning unit to clean the photo sensor based on an amount of light received by the photo sensor meeting a contamination indicating threshold.