We fabricated organic light-emitting diodes (OLEDs) with electron injection layers (EILs) and hole blocking layers (HBLs) made from different sized zinc oxide (ZnO) nanoparticles (NPs) made from indium tin oxide (ITO)/poly(3,4-ethylene dioxythiophene):poly (styrenesulfonate) (PEDOT:PSS)/poly(N,N-bis(4-butylphenyl)-N,N-bis(phenyl)-benzidine) (poly-TPD)/poly(9-vinylcarbazole) (PVK):tris(2-phenylquinoline)iridium(III) (Ir(2-phq)(3))/ZnO NPs/Al. We made OLEDs with ZnO NPs with sizes of 5, 10, and 15nm, and investigated the relationship between the size of the ZnO NPs and the luminance of the OLEDs. The luminance onset voltage is defined as the voltage required to generate a luminance of 1 cd/m(2). The OLEDs with ZnO NPs with sizes of 5, 10, and 15nm had luminance onset voltages of 3.1, 4.1, and 5.3V, respectively, and luminances of the OLEDs with the ZnO NP sizes of 5, 10, and 15nm were, respectively, 455.1, 194.4, and 34.8 cd/m(2) at 8V. The OLED with 5nm ZnO NPs had the lowest luminance onset voltage and highest luminance. This indicates that the Auger electron effect which induces an energy up-conversion process occurred in the OLED with 5nm ZnO NPs.