A method of inputting a user command includes displaying a navigation window including letter key symbols corresponding to keys on a manipulator. If a first user command as to a specific key on the manipulator is input, a specific letter key symbol on the navigation window corresponding to the specific key is activated, and if a second user command as to the specific key is input, the letter corresponding to the activated specific letter key symbol is input. Accordingly, even if the manipulator is separated from the display on which a result of the manipulator is displayed, the user can input a user command by looking at the display only.