Palmitic acid (PA), nano silicon dioxide (nano SiO2), and graphene nanoplatelets (GNPs) were fabricated to composite phase change materials (PCMs) for thermal energy storage. PA acted as PCM, nano SiO2 was used as supporting material. GNP as thermal conductivity promoter was added to modify composite PCM. Nano SiO2 has good adsorption property and can adsorb liquid PCM to prevent leakage. Leakage measurement indicated that PA maximum content in composite PCM is 70 wt%. Chemical and crystal structures, and microstructure of composite PCM were tested by Fourier transformation infrared spectroscope, X-ray diffractometer and scanning electronic microscope, which showed that the raw materials are well mixed by physical action. Differential scanning calorimeter result presented that composite PCM possess phase change temperature at about 60 degrees C and latent heat of 128.42 kJ/kg. Thermogravimetric analyzer and thermal cycle experiment showed that composite PCM have outstanding thermal stability and durability. Thermal conductivity apparatus measurement results indicated that thermal conductivity of composite PCM with 5 wt% GNP is 1.65 times that of composite PCM without GNP. Therefore, this composite PCM are potential materials for thermal energy storage.