Nanoflake-based flower-like CuO nanostructures have been synthesized through thermal decomposition of [Cu(NH3)4]2+ solution without any surfactants and catalysts at low temperature. The products are characterized by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM). The possible formation process based on the aggregation-recrystallization mechanism is proposed. Finally, the obtained flowerlike CuO hierarchical nanostructures have been used as the photocatalyst in the experiments. It is found that the asprepared flower-like CuO hierarchical nanostructures exhibit superior photocatalytic property on photocatalytic decomposition of Rhodamine B due to their hierarchical structures.