Copper nanoparticles (Cu NPs) are a potential material for conductive inks because of their low price, high conductivity, and high electromigration resistance. However, Cu NPs are prone to be oxidized in air, which is lethal to conductivity. In this work, we report on the self-reduction of Cu NPs which are synthesized by the thermal decomposition of copper formate (Cuf) using oleylamine (OAM) as the complexing ligand and stabilizing agent. Although the particle surface partially oxidized to Cu2O in air, the cuprous oxide could be reduced to copper during sintering, because of the release of H-2 through the decomposition of OAM adsorbed on Cu NPs. This self-reduction ability without the help of additional reduction agent makes the Cu NPs a promising material for conductive inks. The feasibility to formulate conductive ink with the prepared Cu NPs is demonstrated.