In this paper, the remarkable improvements in turn-on voltage and luminance have been demonstrated in an organic light-emitting diode (OLED) using cobalt phthalocyanine (CoPc) layer as a hole injection layer (HIL) in a device structure of ITO/CoPc/NPB (60 nm)/Alq(3) (75 nm)/LiF (1 nm)/Al (200 nm). The J-V, L-V and eta(p)-V characteristics were measured at room temperature with a thickness variation of CoPc layer. The improvement of the luminance intensity by a factor of more than two has been obtained and the turn-on voltage at 1 cd/m(2) decreased about I V with the CoPc layer. We propose that such an improvement is mainly due to the relative energy level of the highest-occupied molecular orbital (HOMO) band between that of the NPB hole transport layer (HTL) and the Fermi level of ITO anode, leading to enhance the hole injection from the ITO to organic layer. (c) 2005 Elsevier B.V. All rights reserved.