Disclosed herein are a keypad having a function button used to easily input characters in cellular phones and other character input devices and a method of easily inputting various languages and letters using the function button. The function button is set independently of a menu button. Whenever the function button is pushed in a default character input mode, the default character input mode is changed to an upper-case letter input mode, a lower-case letter input mode, a sentence mark input mode, a numeral input mode, a special character input mode and other language input modes. After a desired character has been inputted in a corresponding changed input mode, the function button is pushed to rapidly return to the initial character input mode. Various sentence marks can be effectively inputted using the function button.