Copper-nanoparticle (CuNP)-filled nanocomposites were prepared with various particle sizes and loadings. The nanocomposites incorporating 20-nm CuNPs with 5 vol % loading displayed optimum properties as determined by electrical, mechanical, and thermal characterization. Silver nanoparticles (AgNPs) with a size of 20 nm were loaded into the epoxy resin to allow a comparison of the properties. Interestingly, at the percolation threshold, a 5 vol % loading of CuNP and AgNP nanocomposites resulted in slightly similar electrical conductiv-ities of 0.01 and 0.02 S/cm, respectively. The CuNP and AgNP nanocomposites were also subjected to thermal aging at 150 degrees C, and we observed that the electrical conductivity of both nanocomposites dropped only by about one order of magnitude after 8 weeks of exposure. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 3145-3152, 2011