Copper nanoparticles are an alternative for the currently used silver and gold nanoparticles in inkjet printing of conductive patterns because of the low cost and high electrical conductivity. However, a serious impediment to using copper nanoparticles for conductive inks is their spontaneous oxidation. This paper describes a simple, large-scale and high-throughput (0.2 M) process for the synthesis of copper nanoparticles in aqueous solution at room temperature in a short reaction period. The well-dispersed antioxidative copper pastes were prepared by dispersing the nanoparticles in an ethanol solution of lactic acid, and then depositing on glass slides. The resistivity of the conductive copper film was 14.0 ± 4.5 μΩ cm after annealing at 200 °C for 30 min in nitrogen, which was about eight times of the resistivity of bulk copper (1.7 μΩ cm). The copper nanoparticles and films were characterized by TEM, UV-vis, SEM, XRD and XPS. Lactic acid-stabilized copper nanoparticles showed relatively better disperse stability and resistance to oxidation than the other carboxylic acids. Furthermore, the copper film after annealing at 200 °C for 30 min in nitrogen showed antioxidative characteristics. It was experimentally proven that lactic acid could react with the copper oxides surrounding copper nanoparticles to form copper lactate, which was then reduced to copper in the annealing process.