Thermally conductive and electrically insulating composite materials are required for thermal management in advanced electronic industry. The present work aimed at creating a composite film of cellulose nanofiber (CNF) and nanodiamond (ND) with superior thermal conductivity. The thermal conductivity of the prepared nanocomposite film was similar to 2.7 Wm(-1) K-1, which corresponds to triple of usual CNF/ND composites with similar composition. The distinct thermal conductivity is attributed to a unique nanostructure we made out in the nanocomposite film. The nanostructure can be characterized by CNF fibrils which are densely covered with ND particles. (C) 2018 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.