It is desirable to improve the performance of phase change materials (PCM)-based thermal energy storage (TES) systems because it increases energy efficiency while decreasing primary energy consumption for low environmental loading. Given that PCM suffers from low thermal conductivity (e.g., due to high latent heat capacity and low phase separation), we investigated several efficient options to improve the thermal conductivity of organic PCM (n-octadecane) based on the addition of (0.1 wt%) expanded graphite (EG). The suspension prepared via ultrasonication led to an improvement of 40% in thermal conductivity corresponding to 0.28 W/m when compared to that of n-octadecane corresponding to 0.20 W/mK. Additionally, the ultrasonication effectively dispersed EG in the PCM for an extended duration relative to the stirring method. A PCM-EG reaction was not evident under ultrasonication conditions. Thus, the thermal conductivity of the PCM exhibited maximum improvement when the two conditions involving extremely low amount of EG and its stable dispersion under sonication condition were maintained. The study demonstrated that a significant improvement in thermal conductivity is obtained with a very low amount of EG and that dispersion stability is obtained even with short ultrasonication times.