Self-assembled three dimensional (3D) hierarchical, flower-like iron oxide micro/nano materials, including pure Fe3O4, alpha-Fe2O3/Fe3O4 composite and pure alpha-Fe2O3, were developed to degrade hexachlorobenzene (HCB) at 300 degrees C. It was found that pure Fe3O4 exhibited the highest activity with the reaction rate constant at 0.959 min(-1). Identified degradation products with lower chlorinated benzenes suggest a stepwise hydrodechlorination was occurring for the degradation of HCB on the synthesized Fe3O4. The characterization of the synthesized iron oxides after reaction indicates the Fe3O4 phase was partially transformed into alpha-Fe2O3 phase and in return accelerated progress of hydrodechlorination of HCB. The superior performance of as-prepared Fe3O4 could be attributed to its structural feature such as surface area and pore structure, as well as its phase transformation during HCB degradation.