PCM (phase change material) is one of the most popular media in TES (thermal energy storage) system for its high energy storage density and suitable melting temperature. However, its low heat transfer performance leads to low rate of heat stored and released. In this paper, a highly thermal conductive additive of ENG-TSA (expanded natural graphite treated with sulfuric acid) had been used to improve the thermal conductivity of the KNO3/NaNO3 nitrate mixture. Composite samples with different densities and mass fractions of ENG-TSA were produced by the cold-compression method. The thermal conductivities, phase transition properties, thermal stability and micro structures of different samples were evaluated, and results showed that the highest effective thermal conductivity of composite PCM was 50.78 W/(mK), which was 110 times higher than that of salt powder. The additive of ENG-TSA in the PCM caused a slight decrease in latent heat, but there was no remarkable variation in phase change temperature. The salt segregation from the consolidated composite PCM blocks could be controlled at a small value when the densities and mass fractions of ENG-TSA were optimized. (C) 2014 Elsevier Ltd. All rights reserved.