Solution-processed nickel oxide (NiO) nanoparticles (NPs) were used as a hole injection layer (HIL) to enhance the performance of quantum dot light-emitting diodes (QLEDs). NiO NP solution was synthesized by a sol-gel method. We synthesized NiO NPs of sizes 10 nm, 20 nm, and 30 nm by controlling the annealing temperature of an NiO NP powder-like precipitate. The different-sized NiO NPs were applied on QLEDs as HILs, and exhibited good hole injection properties. All layers of the QLEDs except the electrodes were fabricated using a solution process. The QLED with 10 nm NiO NPs had the lowest turn-on voltage of 2.5 V, indicating that NiO NP HIL based on 10 nm NPs reduced the energy barrier for hole injection into the hole transport layer (HTL) most. The maximum luminance and luminous efficiency of the QLED with NiO NPs of 10 nm were 17,818 cd/m(2) and 7.11 cd/A, respectively.