Flower-like ZnO microstructures are prepared by a simple one-step hydrothermal process at 85 °C for 30 min. ZnO hierarchical structures about 3.5 μm in size are assembled by rod-like crystals with sharp ends. The growth of flower-like ZnO is rapid. OH⁻-driven oriented aggregation and multistep nucleation result in the formation of flower-like ZnO structures. The products exhibit excellent photocatalytic performance in the degradation of methyl orange under UV-light irradiation, which can be attributed to their assembled open structures.