The photometric, electrical, thermal and life features of LEDs are highly dependent on each other. All these factors should be considered together in order to optimize the operating point of LEDs. A photo-electro-thermal-life theory about LEDs is presented. This theory shows the inner links among photometric, electrical, thermal and life features of a LED. Using this theory, one can build a life prediction model and find out the relationship between output luminous flux and lifetime of LED. Based on this relationship, one can predict the lifetime of a LED and find the proper operating point, at which the LED will generate the maximum amount of luminous flux in its total life cycle.