We have investigated in detail the dependence of the electroluminescent characteristics on the thin LiF layer inserted under the electrode layer of multi-layered PVCz polymer light emitting diode (PLED). Clear dependency has been obtained and reduction in the operating voltage, improvement in luminance and the luminous and current efficiencies have been realized with the optimization of the thickness. The effect of thin LiF layer has been analyzed by using the current voltage dependence at low voltage regions and the current is supposed to be carried considered LiF layer by tunnel injection between the PVCz layer and the electrode layer. (C) 2004 Published by Elsevier B.V.