Boehmite (g-AlOOH) has demonstrated to have superior performance as coating material for battery separator, especially lithium-ion battery, due to its excellent wetting ability and thermal stability. The products of Bayer process from bauxite are typically in the form of aluminum trihydrate (ATH). Observing the phase diagram of Al2O3-H2O, it was assumed that the ATH might be converted into boehmite by controlling the pressure and temperature under hydrothermal condition. Therefore, the purpose of this work is to investigate the effect of operating conditions on the conversion of ATH to boehmite in a hydrothermal reactor. The experiments were carried out in a 100-mL hydrothermal reactor with a liquid filling volume of 30% and 50%. The reaction time was varied from 2 to 8 h. The hydrothermally treated ATH particles were characterized by an X-ray diffraction (XRD) and a scanning electron microscopy (SEM). The XRD patterns exhibited that the ATH has not been converted yet into boehmite at a reaction time of 2 h for all conditions. When the reaction time was prolonged to 4 h, ATH was transformed into boehmite at filling volume of 30% and 50%. It appears that liquid filling volume influences the transformation of ATH into boehmite. As observed by the SEM images, the morphology of particles also changed by the change of the crystalline phase. The particles changed from nearly spherical-shaped to cube and the particle size was typically larger