This work presented the ozone (O3)-hydrothermal process to prepare expandable graphite (EG), which is one kind of halogen-free flame retardant. The results showed the expanded volume of EG using the O3-hydrothermal process (O3-HEG), which is higher than those compared with convectional liquid phase synthesis, ultrasound irradiation, and hydrothermal method. Raman spectra and energy dispersive x-ray were used to analyze the structure and confirm that the EG had been prepared. Scanning electron microscope was utilized to observe the morphology of EG and expanded graphite. Thermogravimetric analysis shows that EG can slowdown thermal degradation of polymer and improve the thermal stability of composites. Limiting oxygen index value of composites with 40 wt% loading of O3-HEG is 33, which is higher than that of high-density polyethylene (HDPE)/natural graphite composite. HDPE composites-containing 30 wt % O3-HEG is capable of passing the V-0 classification and has an anti-dripping behavior. © The Author(s) 2013.