TiO_2 achieved by the Sol-gel method are inserted in organic EL devices as a buffer layer.The effect of the different TiO_2 thickness on the device with the structure ITO/TiO_2/NPB/Alq_3/LiF/Al.When the buffer layer is 2 nm thickness,the voltage of the devices decreases and the current density of the devices increases,because of the tunneling effect,and the efficiency of the device is best at the buffer layer thickness of 6 nm,because of the balance of the injecting hole and electron.As the current density is 100 cd/mA~2,the efficiency of the 6 nm-thickness buffer layer device is 5 cd/A,and that without buffer layer device is 3.45 cd/A.The smooth of the ITO surface is also a factor that the efficiency increases.