A novel flower-like γ-AlOOH and γ-Al₂O₃ with hierarchical nanoarchitectures were successfully synthesized via the assembly of P123 and aluminium chloride with urea being used as a precipitating agent under hydrothermal conditions. The influence on morphology control was studied by altering the amount of P123 (V_p123) and molar ratio of CO(NH₂)₂/Al³⁺ (R_UAl), implying that the increasing R_UAl could lead to the crystal growth in the form of nanofilms or nanostrips and the addition of P123 could have a morphology-alteration impact on the 3D hierarchical architectures, and the possible formation mechanism was carefully studied. Further adsorption investigation indicates that the flower-like γ-Al₂O₃ exhibits an enhanced adsorption performance towards methyl orange as a result of the unique hierarchical structure, and the addition of P123 also has an drastic effect on the surface acidity of the as-prepared flower-like γ-Al₂O₃.