A ZnO nanoparticle layer was synthesized by the sol-gel method on ITO glass. Devices with a ZnO nanoparticle layer/polymethylmethacrylate (PMMA) heterojunction structure were then fabricated and the room temperature electroluminescence (EL) spectra were studied. Under DC bias, ultraviolet (UV) EL at 390 nm from the ZnO band edge emission was observed, and defect-related emission of ZnO at 600 and 760 nm were also detected. The EL mechanisms are discussed in terms of the carrier tunneling process.