In this study, ZnO nanostructures with different morphologies including nanorods, nanowires, and nanobrushes were synthesized by a simple hydrothermal process without using any structure-directing reagent. The samples were characterized by X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy. The influence of the preparation parameters on the morphology of ZnO is discussed. Gas-sensing properties of the materials were investigated. The results reveal that all the prepared nanostructured ZnO powders show high response to ethanol, among which, the three-dimensional nanobrushes show the highest response, demonstrating excellent potentiality for ethanol sensors.