Poly(methyl methacrylate)/hydroxyapatite nanorod (PMMA/HANR) composites were synthesized by in situ polymerization of methyl methacrylate (MMA) in a suspension of HANRs in ethanol. It was found that the thermal properties of the composites were improved effectively by increasing the inorganic HANR loading, especially when the amount of loaded HANRs was high enough (>= 9.3 wt %). The experimental results showed that graphitized char was generated in the residue after thermal degradation of the composites in air at 550 degrees C for 0.5 h. The residues of the composites with higher inorganic loadings formed a compact and continuous network structure made up of inorganic fillers and carbonaceous char. This indicates that the carbonization; behavior of HANRs in PMMA matrix leads to improvement of the thermal properties of the polymer.