The boehmite (Al 2 O 3 ·H 2 O) hierarchical nanostructure with spindle-like morphology has been successfully synthesized via ionic liquid-assisted hydrothermal synthetic method under mild condition using an ionic liquid 1-butyl-3-dimethylimidazolium bromide ([Bmim][Br]) as a template. The proposed formation mechanism has been investigated and the hydrogen bond-co-π-π stack mechanism is used to be responsible for the present formation of the precursor hierarchical nanostructure. The γ-Al 2 O 3 hierarchical nanostructure was obtained by calcining the as-synthesized precursor at 500 °C for 2 h, preserving the same morphology.