A scanner capable of selecting scanning positions includes a scanning window, a scanning module and a control processing unit. Before a sheet of documents is scanned, the control processing unit moves the scanning module to a first location to generate a first image datum, and moves the scanning module to a second location to generate a second image datum. The control processing unit compares both the image data according to differences between the first and second image data, and moves the scanning module to a scanning location for scanning the sheet of document.