Nanodiamonds (NDs) have attracted great interest due to their high refractive index and thermal conductivity. The unique properties of NDs, including high thermal conductivity, chemical stability, and tensile strength, make them a promising candidate for enhancing OLED efficiency. NDs were coated either beneath or above the PEDOT:PSS hole injection layer in a green-emitting OLEDs (ITO/ PEDOT:PSS/NPD/CBP:Ir(ppy)3/TPBi/Liq/Al). The best performance is obtained from the device with NDs layer beneath the PEDOT:PSS layer where the current efficiency (CE) is increased by 5.4%, power efficiency (PE) is increased by 17.7% and the external quantum efficiency (EQE) is increased by 4.5% at a luminance of 10000 cd/m2 as compared to a standard device without NDs layer.