Cu nanoparticles (NPs) are widely used in numerous applications because of their unique optical, catalytic, and electrical properties. However, the synthesis of monodisperse Cu NPs often involves harmful reducing agents under an inert atmosphere in a solution with low Cu ion concentrations. In this study, monodisperse Cu NPs were synthesized by the thermal decomposition of an oleylamine-coordinated Cu oxalate (Cu(ox)) complex (OA-Cu(ox)) in oleylamine in air with no reducing agent. The structure of OA-Cu(ox) was analyzed using Fourier-transform infrared spectroscopy. The decomposition temperature of the oxalate ion was reduced to 130 °C from that of Cu(ox) (300 °C) because of the coordination of the primary amino group of oleylamine with the Cu ion of Cu(ox). Furthermore, the decomposition temperature of OA-Cu(ox) strongly affected the size distribution of the synthesized Cu NPs. Monodisperse Cu NPs were successfully synthesized by the thermal decomposition of OA-Cu(ox) at temperatures higher than 240 °C with high yield (∼90%). The synthesized Cu NPs were readily dispersed in toluene.