We report a hydrothermal synthesis of ZnO nanoparticles, nanoplates and nanoflowers, and investigate their gas-sensing behaviors. In addition, we also explore the effect of the concentration of surfactant CTAB on the ultimate morphology of the nanoflowers and propose the growth formation mechanism. Display omitted •ZnO nanoparticles, nanoplates and nanoflowers were successfully synthesized.•The nanoflowers architectures showed the best gas-sensing properties.•CTAB played a vital role in the ultimate morphology of the 3D hierarchical architectures. ZnO nanoparticles, nanoplates and nanoflowers have been successfully synthesized via a facile hydrothermal route, and their microstructures and gas-sensing properties to ethanol were investigated. Among all the nanostructures, the nanoplates-assembled nanoflowers exhibited significantly higher gas-sensing performances than the others, which may ascribe to their hierarchical architectures with large specific area and abundant spaces for gas diffusion. Furthermore, we surprisingly found that the concentration of surfactant CTAB used had an essential effect on the ultimate morphology of the hierarchical nanoflowers. We hoped our findings could be in favor of further investigations on the fabrication of perfect hierarchical architectures.