An MoO3 buffer layer prepared by thermal evaporation as hole injection layer was investigated in organic light emitting diodes. The MoO3 film inserted between the anode and hole transport layer decreased the operating voltage and enhanced power efficiency. Introduction of 1 nm MoO3 film, which was found to be the optimum layer thickness, resulted in 45% increase in efficiency compared with traditional ITO anode. Results from atomic force microscopy and photoemission spectroscopy showed that smooth surface morphology and suitable energy level alignment of ITO/MoO3 interface facilitated hole injection and transport. The hole injection and transport mechanism at the ITO/MoO3 interface in thin and thick buffer layers were analyzed.