Organic EL devices using ZnO as a hole-injecting buffer layer have been prepared. The structure of the device is ITO/ZnO/NPB/AlQ/Al. The insertion of this thin film with higher work function can improve the ITO surface and reduce drain current, which makes the device much steady and have a higher break-down voltage. ZnO buffer layer can also block the hole injection at the interface of anode and hole transporting layer,which can be attributed to the more balanced carriers concentration in emissive layer and raise the EL efficiency. The EL efficiecy of device with ZnO buffer is about 35% at 7 V which is higher than that of the device without it.