Cu nanoparticle ink was prepared from Cu nanoparticles that were coated with a gelatin layer at an average diameter of 46 nm. The Cu nanoparticle ink was applied on the polyimide substrate. Conductive films were fabricated using the Cu nanoparticle ink with a two-step annealing process consisting of oxidative pre-heating at 200 degrees C under 10 ppm O-2-N-2 mixed gas flow and reductive calcination at 250 degrees C under 3 vol.% H-2-N-2 mixed gas flow showed a low resistivity of 5 mu Omega cm. The hydrolysis of the remaining gelatin layer by H2O vapor, which was formed during the reduction of the Cu oxide by 3 vol.% H-2-N-2 mixed gas, was suggested. The results suggest the possibility of the removal of the gelatin layer without oxidative pre-heating and simultaneous sintering of Cu nanoparticles in reductive calcination. (C) 2011 Elsevier B.V. All rights reserved.