Zinc Oxide (ZnO) hierarchical microstructures with diverse morphologies were prepared through a simple one-step hydrothermal method with zinc chloride and sodium hydroxide as source materials, and citric acid as the modifying agent. The structure and morphology of the samples were studied through X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, and Brunauer-Emmett-Teller specific surface area determination. The morphology of the samples was easily regulated by changing the concentration of citric acid. Branchy structures, porous flowers, porous microspheres, solid flowers, solid flower spheres, and solid spheres of ZnO were obtained. The porous structures obtained with 150-200 mmol Á L À1 citric acid showed excellent photocatalytic activities for the degradation of methylene blue under UV irradiation. A possible regulation mechanism of citric acid in ZnO crystals was proposed based on the experimental results.