Provided are a method of inputting a user input by using a mobile device and a mobile device using the method. The method of performing operations of the mobile device according to a plurality of input modes, the method being performed by the mobile device, includes operations of, determining whether the mobile device is placed on a surface, if the determining of whether the mobile device is placed on the surface indicates that the mobile device is placed on the surface, changing an input mode of the mobile device, and performing a preset operation that corresponds to the changed input mode.