Hierarchical ZnO/ZnFe₂O₄nanoforests with ZnO backbones and ZnFe₂O₄nanosheets were successfully prepared by a facile two-step process. The products were fabricated by immersion of the as-synthesized ZnO nanorod arrays in a 0.2 M aqueous solution of ferrous sulfate and subsequent calcination at 500℃. Various techniques were employed for the characterization of the structure and morphology of the hybrid nanostructures. The results showed that a high density of ZnFe₂O₄nanosheets was planted on the surface of the ZnO nanorods. Interestingly, the morphologies of the ZnO/ZnFe₂O₄hierarchical nanostructures can be tailored by changing the concentration of the FeSO₄solution and the immersion time. A possible formation process and growth mechanism was proposed as the ZnO nanorods were partly dissolved during the immersion period. In order to demonstrate the potential application of the nanorods, two sensors based on bare ZnO and ZnO/ZnFe₂O₄composites were fabricated and their gas-sensing properties were investigated. The results indicated that the obtained advanced ZnO/ZnFe₂O₄nanostructures exhibited enhanced sensing properties to ethanol compared to the primary ZnO nanorods. For example, upon exposure to 100 ppm ethanol, the response of the hierarchical ZnO/ZnFe₂O₄composites was about 4 times higher than that of the primary ZnO nanorods at the operating temperature of 275℃.