Hierarchical CuO microspheres have been successfully synthesized by a facile one-step hydrothermal method. Morphology analysis indicated that the as-obtained hierarchical CuO microspheres with diameters of ∼2.3 μm were assembled as monocrystalline nanosheets. The three-stage growth process from Cu(OH)₂ to the final hierarchical CuO microspheres was studied in detail. Moreover, the gas sensing properties of the as-obtained materials for ethanol were studied. The results reveal that the hierarchical CuO microspheres based sensors exhibit good selectivity, high sensitivity as well as rapid response (2 s) and recovery (8 s) time for ethanol, all of which are attributed to their characteristic hierarchical structures assembled with monocrystalline nanosheets.