In the present work, the thermal energy storage phase change materials (PCM) based on paraffin/high density polyethylene (HDPE) composites were prepared by using twin-screw extruder technique. The morphology and properties of the PCM composites based on the flame retardant system with expanded graphite (EG) and ammonium polyphosphate (APP) were characterized by Scanning electron microscope (SEM), Differential scanning calorimeter (DSC), Thermogravimetric analyses (TGA) and Cone calorimeter tests. It was observed from SEM images that paraffin dispersed well in the three-dimensional net structure formed by the HDPE. The SEM images also indicated that the EG and APP were well dispersed in the PCM composites. The DSC measurements indicated that the additives of flame retardant had little effect on the temperatures of phase change peaks and thermal energy storage property. The TGA results showed that the loadings of the EG and APP increased the temperature of the maximum weight loss and the charred residue of the PCM composites at 650 degrees C, contributing to the improved thermal stability properties. It was revealed from the Cone calorimeter tests that the peak of heat release rate (PHRR) decreased significantly. To further investigate the synergistic effect between the EG and APP, it was observed from SEM images that the homogeneous and compact charred residue structure after combustion contributed to the enhanced thermal stability, improved flammability and increased self-extinguishing properties of the PCM composites. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.