A novel and facile hydrothermal approach was developed for the synthesis of three-dimensional flower-like alpha-Fe2O3 nanocrystals grown directly on Fe substrate using Fe(NO3)(3) and hexamethylenetetramine as precursors in aqueous medium. A reaction mechanism for the synthesis and growth of the flower-like alpha-Fe2O3 nanocrystals was proposed to explain the effects of reaction time and temperature on the morphology and structure of the as-synthesized nanocrystals. Moreover, cube-like shapes of Fe2O3 nanocrystals were produced when urea used as hydrolysis reagent during the synthesis process. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction were employed to confirm the unique morphology, high crystallinity and pure phase structure. The as-synthesized flower-like alpha-Fe2O3 nanocrystals displayed a higher activity and C2-4 olefin/paraffin ratio in Fisher-Tropsch synthesis than the cube-like shape of alpha-Fe2O3 nanocrystals, possibly due to their high specific surface area as well as diversity of catalytic active sites. The produced flower-like Fe2O3 nanocrystals sample displayed high catalytic activity and C2-4 olefin/paraffin over the cube-like Fe2O3 nanocrystals for Fischer-Tropsch synthesis, due to the high specific surface area and diversity of catalytic active sites.