A surfactant-free strategy for controllable growth of various hierarchical CuO nanostructures was successfully achieved through a facile, one-pot solution-phase transformation of Cu_xM_y(OH)_z (M = SO₄²⁻, Cl⁻, NO₃⁻ and CH₃COO⁻) precursors method without extra heat treatment in air. The formation of CuO nanostructures here is essentially determined by the species of Cu_xM_y(OH)_z precursors. The nanoparticle aggregation with controllable shapes can be formed under different concentrations of reactants. Hierarchical CuO nanoflowers with different branched building blocks were prepared by the transformation of Cu₄(SO₄)(OH)₆ precursors. Hierarchical CuO nanospindles and nanoplates were fabricated by the transformation of Cu₂(OH)₃NO₃ precursors. Hierarchical CuO nanoplate assemblies and nanoplates were synthesized by the transformation of Cu₇Cl₄(OH)₁₀H₂O and C₈H₆Cu₃O₁₀4H₂O precursors, respectively. Structural and morphological evolutions were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and field-emission scanning electron microscopy (FESEM). This study is of great importance in controllable synthesis of surfactant-free CuO nanostructures, because it not only enriches the family of CuO architectures but also offers a good chance to understand the nature of the solution-phase precursor transformation method for synthesizing CuO nanostructures. Significantly, it is believed that this surfactant-free solution-phase precursor-driven synthesis of nanostructures presented here might provide a green approach to design and growth other well-defined metal oxide nanostructures.