CuO nanoleaves with a mesoporous structure were formed in an aqueous solution of triethylamine at room temperature. The growth process of the CuO nanoleaves in a triethylamine solution was investigated by varying the reaction time. It is shown that the CuO nanostructures form by reconstructive transformation from Cu(OH)₂, going through a 0D nanoparticle → 1D nanowire → 2D nanoleaf dimensional transition process. The mechanism for the amine-induced formation of CuO at room temperature was studied by using different aliphatic amines. It is revealed that the amines play multiple roles on CuO formation, i.e. acting as an alkali, dominating the Cu(OH)₂ to CuO transformation, and directing the oriented crystal growth of CuO. This route is simple, rapid, involves no additional alkalis or directing agents, and can proceed at room temperature. The as synthesized CuO exhibits excellent catalytic activity for cyclohexene oxidation with oxygen under solvent-free conditions.