Devices with a structure of ITO(indium tin oxide)/TPD(N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine)/Alq3[tris-(8-hydroxy-quinolinato) aluminum)/Liq(8-hydroxy-quinolinato lithium)/Al, here Liq was used as an electron injection layer,were investigated. Effect of Liq thickness on the electroluminescent efficiency and characteristics of current density-voltage was investigated. Experimental results show that a device with 0.5 nm thick Liq as an electron injection layer was the highest in the efficiency which was about five times higher than that without Liq. This may be attributed to the enhancement of electron injection from the introduction of Liq, which generate a dipole layer and form an ohmic contact between aluminum electrode and Alq3 organic layer. Increase the thickness of Liq layer leads to the decrease of device efficiency and the increase of operating voltage. Comparing with same structure devices using LiF as an injection layer, the trend of change of device performance with the thickness increase of injection layer was similar, however the devices using Liq as an injection layer are less sensitive to the thickness of injection layer. This is because that LiF is an insulator and Liq is a semiconductor. This is very important for use of a large size substrate in the mass production.