Abstract: A pen with illuminating function is disclosed. The pen has a dimension as that of a general pen so that a user can easily hold the pen to write. A bulb or an LED is disposed in a pen head of the pen and an adjusting assembly is disposed in a lower pen sleeve for adjusting the position of a pen core and electrically connecting with a battery contained in an upper pen sleeve. A switch assembly is disposed in an upper end of the upper pen sleeve, whereby an adjusting button of the switch assembly is rotatable to make a conductive projecting board contact with or disengage from a conductive ring member so as to close or open a circuit and turn on or off the bulb or LED. The pen provides an illuminating effect for the user to write in a dim place while enabling the user to write without obstruction.