A scanning method of an image forming apparatus including free-scanning a page to be scanned, setting a scan data generation area to generate scan data on the free-scanned image, if a subsequent page is main-scanned, detecting a relative position of the scan data generation area from a scanned area of the main-scanned page, adjusting the scan data generation area using the detected relative position, and generating scan data for the adjusted scan data generation area, and an image forming apparatus to perform the same. Therefore, a plurality of pages can be precisely scanned or copied with a specific size.