A facile additive-assisted complex-precursor solution method is demonstrated for one-pot assembly of well-defined hierarchical Cu₂O nanospheres with tunable sizes and shapes. Structural and morphological evolutions were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and field-emission scanning electron microscopy (FESEM). The formation mechanism of these hierarchical nanostructures was rationally proposed, which can be attributed to the synergic effect of nanoparticle-aggregation and ripening with facet-selective adsorption. Significantly, it is believed that the additive-driven assembly of hierarchical architectures reported here would provide a facile way to design other exciting metal oxide nanostructures with controllable sizes. The photocatalytic superiority and stability for the methyl orange (MO) photodegradation by the hierarchical Cu₂O nanospheres could be attributed to their exposed high active facets, which would be quite feasibly used for application in the fields of photocatalytic hazard pollutants.