ZnO nanorod arrays decorated with NiO nanosheets on FTO substrates were prepared via a simple ultrasonic spray pyrolysis process combined with chemical bath method. The synthesized samples were characterized and analyzed by scanning electron microscopy and X-ray diffraction. The hierarchical and porous morphologies of ZnO/NiO core-shell nanoheterojunctions could be controlled by changing the growth time of NiO sheets. The "oriented attachment" and "self-assembly" crystal growth mechanisms were proposed to explain the formation of the ZnO/NiO nanostructures. Sensors based on the ZnO/NiO heterojunction nanostructure were fabricated and investigated for their ethanol-sensing properties. The result indicated that response was about 180% toward 100 ppm ethanol at an operating temperature of 200℃. The growth approach in this work offers a new technique for the design and synthesis of transition metal oxide hierarchical nanoarrays which are promising for gas sensing applications.