A function input method for providing in advance to a user information on a function to be executed when the user presses a button to execute a specific function in a portable terminal, such as a cellular phone, and a button input apparatus using the same. The method includes: receiving a button input when the user presses one of a plurality of buttons; determining whether a mode of the button input is a touch mode or a click mode; and displaying a preview image of a function corresponding to the pressed button if the button input mode is the touch mode and executing the function corresponding to the pressed button if the button input mode is the click mode.