Morphology-controlled zinc oxide (ZnO) structures, including micrometer-sized rods, nanorods, urchin-like structure, flower-like structure, and fan-shaped structure, have been synthesized by a hydrothermal method without the use of a surfactant or an amine salt. The morphology control has been achieved by simply varying the solution pH value. The precursor used in the hydrothermal process was zinc carbonate hydroxide hydrate (Zn4CO3(OH)6·H2O). The solution pH value was varied through the addition of NaOH(aq) or HCl(aq). The mechanisms for the formation of different morphologies are discussed. Despite the difference in the morphology, the resulting ZnO nanostructures all exhibit violet emission from 379.7 nm (3.27 eV) to 404.9 nm (3.06 eV), which suggests the application of the ZnO nanostructures in violet light-emitting diodes.