A composite phase change material (PCM) containing Cu nanoparticles (NPs) was prepared. XRD and TEM results showed that the Cu NPs were pure metal with irregular morphology. The diameter of Cu NPs was in the range of 5-15 nm. The composite PCM melted at the temperature of 310 K and the total phase change enthalpy was 122 J/g. The thermal conductivity of the composite PCM was reduced by Cu NPs from 0.130 W/mK to 0.102 W/mK. The as-prepared composite PCM might be an appropriate candidate for thermal protection where PCM with lower thermal conductivity is needed.