As a phase change material (PCM), paraffin has high latent heat, but low thermal conductivity. In this study, expanded graphite (EG) was added into paraffin to form composite material with both high thermal conductivity, and high latent heat. Moreover, the EG was organo-modified to improve the compatibility of EG and paraffin. The results reveal that thermal conductivity of paraffin (0.30 W/mK) was increased to 8.377 W/mK by adding 30% mass fraction of modified EG. The results also indicated that the heating and cooling times of modified EG/paraffin composites were increased significantly with respect to that of pure paraffin. The experimental results revealed that the use of modified EG can be considered an effective method to improve the thermal conductivity of paraffin.