The microstructure of carbon nanotube reinforced copper matrix (CNT/Cu) nanocomposites, processed by molecular-level mixing, exhibits CNTs homogeneously dispersed in the Cu matrix. Measured thermal conductivity of the fabricated CNT/Cu nanocomposites decreased as the CNT volume fraction increased from 5% to 10%. A low coefficient of thermal expansion (CTE) was obtained from the 10 vol.% CNT/Cu nanocomposite. These results indicate that embedded CNTs provide CNT/metal interfaces as thermal barriers but also stiffen the metal matrix composites. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.