Hexagonal, bullet-like ZnO microstructures and nanorod arrays have been prepared successfully by a simple hydrothermal method without surfactants. The structure and composition of the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectrum (EDS), infrared spectrum (IR), and UV-vis diffusive reflectance spectrum (UV-vis). The as-synthesized bullet-like ZnO has hexagonal microstructures with an average diameter of about 10 mu m and a length of 30 mu m. The obtained ZnO nanorod arrays are highly oriented. Their average diameter is around 50-100 nm, and their length is about 1 mu m. In the present work, the effect of concentration, heating temperature, and time on the morphology of ZnO structures was studied experimentally, and their formation was discussed in detail. The photocatalytic degradation of methylene blue (MB) was also investigated.