Abstract: Novel hexagonal prism polycrystalline γ-Al₂O₃ particles were hydrothermally synthesized in ammonia medium by using AlCl₃·6H₂O as aluminum precursor. The γ-Al₂O₃ particles were characterized by means of XRD, SEM, TEM and nitrogen sorption measurements and the formation mechanism of crystal morphology was investigated. The results indicated that after the hydrothermal treatment of aluminum precursor in ammonia medium as well as the later calcination process, γ-Al₂O₃ particles with perfect hexagonal prism shape can be obtained; the border size and length of the hexagonal prism are about 0.3 and 2.5 μm, respectively. TEM images show that the hexagonal prism γ-Al₂O₃ particles are aggregated with fine particles of about 10 nm and then take the polycrystalline gamma phase. Such a γ-Al₂O₃ material exhibits a BET area of 274 cm²/g and pore volume of 0.51 cm³/g with a narrow pore size distribution centered at 5.5 nm. The results also suggest that the formation of the hexagonal prism γ-Al₂O₃ particles is closely related to the arrangement of the steady AlOOH precursors interacted with NH⁺₄ ions.