We investigated the carrier injection properties in small molecular organic light emitting devices with a mixed single layer by measuring current-voltage characteristics using Au/MoO3 as hole and Al alloy as electron injection electrodes. The carrier injection mechanism at the interface was primarily ascribed to the Schottky thermionic emission with the barrier height of 0.25 eV for holes and 0.67 eV for electrons. It was found that the barrier height of the electron was dependent on LUMO level of electron transport material. In addition, the interfacial state analysis demonstrated that the electron barrier height was also dependent on the interfacial conditions of the devices. (C) 2010 Elsevier B. V. All rights reserved.