In this study, the dependence of metal/Alq3 Schottky contact barrier on the current¡Vvoltage characteristics of organic light emitting diodes was investigated to know the charge injection mechanism of OLED with the single-layer metal/Alq3/ metal structures, and the current density increases obviously with the reduction of contact potential barrier. As the thin LiF layer is inserted between the Al electrode and the Alq3 layer, it shows that the electron injection was promoted, and higher electroluminescence efficiency was also obtained. Both the energetic barrier and the tunneling integral parameter are reduced when the LiF layer thickness increases. For very thin films of LiF, the beneficial effect of the barrier reduction is dominant. When the film grows thicker, the negative insulating effect becomes dominant. Besides of simulating the current¡Vvoltage characteristics of organic light emitting diodes (OLED) based on Alq3 in combination with different cathodes, it was simulated that the current¡Vvoltage characteristics of OLED with an inserted LiF layer between metal and organic material, and then the OLED with various thicknesses of LiF films were also simulated. Finally, the result of simulations was compared to achieve a better description for the characteristics of current¡Vvoltage for the single carrier and layer based OLED.