A system and method for illumination brightness of a head up display for a vehicle are provided. The method includes turning on, by a controller, the vehicle and the head up display and determining whether an auto light is operated. The head up display is illuminated when the auto light is normally operated. The controller determines whether a tail lamp mounted within the vehicle is operated when the auto light is abnormally operated. The head up display is illuminated as a night condition when an exterior night condition is detected and the tail lamp is normally operated. Additionally, the controller determines whether a tail switch is operated when the tail lamp is abnormally operated. The head up display is illuminated as a daytime condition when an exterior daytime condition is detected and the tail switch is abnormally operated.