Large-scale aggregative flowerlike ZnO nanostructures, consisting of many bunches of nanorods at different orientations with a diameter of about 60 nm and a length of 1 µm, have been synthesized through a simple hydrothermal process at a lower temperature. The x-ray power diffraction pattern indicates that the novel flowerlike ZnO nanostructures are hexagonal, and the selected area electron diffraction reveals that the ZnO nanorods are single crystal in nature and preferentially grow along [0 0 1]. Raman spectrum, room-temperature photoluminescence and UV–vis absorption spectra are also discussed. Furthermore, the influence of the reaction time on the morphology of the ZnO nanostructures is investigated, and a possible growth model is proposed. Finally, the gas sensor based on the ZnO nanostructures exhibits high sensitivity for ethanol as well as quick response and recovery time due to the high surface-to-volume ratio.