Self-assembled γ-Al2O3 with hierarchical structure was successfully obtained via thermolysis of γ-boehmite (γ-AlOOH) particles, which was hydrothermally derived from aluminum ammonium sulfate hydrate (NH4Al(SO4)2·12H2O), urea, poly-glycol (PEG)-2000, and deionized water. SEM observations indicate that the as-synthesized γ-AlOOH has hierarchical flower-like structure, composing of needle-like building blocks. After calcinations at 800°C, it converts to cubic γ-Al2O3 with hierarchical structure retained by a topotactical process. N2 adsorption and desorption reveal that the obtained γ-Al2O3 has a BET surface area of 101 m2/g with a narrow mesoporous size of about 13 nm and a broad macroporous-size distributions of 200–500 nm, respectively. The as-generated γ-Al2O3 with hierarchical structure shows good capacities for removing Congo red from wastewater, indicating that 3D hierarchical structure has excellent adsorption ability.