Monodisperse alpha-Fe₂O₃discoid crystals have been prepared through a hexamethylenetetramine (HMT)-assisted hydrothermal process combined with subsequent acid-dissolution. First, uniform alpha-Fe₂O₃round-edged hexahedrons with a size of about 1.2 mu m were synthesized. Subsequently, by a controlled acid etching process, the as-obtained alpha-Fe₂O₃uniform hexahedrons could be facilely transformed into monodisperse alpha-Fe₂O₃discoid crystals, without influencing the original crystal phase. Both field emission scanning electron microscope results and transmission electron microscope results revealed that the "discuses'' were made of piled up nanoparticles. The selected area electron diffraction pattern from the whole discoid crystal displayed that all the nanoparticles were highly oriented, which resulted in the single-crystal "discus'' features. To demonstrate the usage of such alpha-Fe₂O₃discoid crystals, the obtained sample was applied to fabricate a gas sensor which was then tested for sensitivity to three kinds of gases (ethanol, methanol and acetone). The results of the test showed that the sensor had a high level of response and good recovery characteristics towards ethanol at the operating temperature of 238℃.