An image capturing apparatus for capturing images of a document includes a scan module, a control unit and a document feeder. The scan module is for scanning the document while the document feeder is for feeding the document to the scan module. The document feeder includes a paper tray, a sheet feeding mechanism, and a roller set. The paper tray is for storing the document, the sheet feeding mechanism is for moving the document, and the roller set is disposed on the paper tray. When the document is fed from the paper tray by the sheet feeding mechanism, the control unit determines a size of the document according to the rotating status of the roller set.