A model for calculation the built in potential (V bi ) in organic light emitting diodes (OLED) is presented.The voltage at which the current underillumilation in OLED is equal to the dark current,namely the net photocurrent is zero,which we term the "compensation" voltage (V 0).The relation between the V bi and V 0 is maintained.It is dependent of the temperature and material parameters.At low temperature,V 0 is equeal to V bi .However,because of diffusion of thermally injected charges at room temperature,V 0 is lower than V bi .The theory derived from this model consists with the experimental results.