An image forming apparatus has a structure that it is able to clean an ACR sensor in accordance with an ACR operation, and without requiring a user manipulation. The image forming apparatus includes a plurality of photosensitive media on which color images are formed respectively, a transfer unit to receive color images from the plurality of photosensitive media and transfer the received color images onto a printing medium, an auto color registration (ACR) unit to perform the ACR operation with respect to the transferred image, and a cleaning member to clean the ACR unit in association with the ACR operation. A method of cleaning an ACR sensor in an image forming apparatus is also provided.