Hierarchical and hollow cylinder comb-like nanostructures with ZnO backbones and SnO₂branches were successfully prepared by an ultrasonic spray pyrolysis process. The observation of field scanning electron microscopy and transmission electron microscopy showed that a high density of SnO₂nanowires grew epitaxially on as-synthesized ZnO nanorod arrays. The diameter and length of the SnO2 nanowires were about 50 nm and 300 nm. The morphologies of ZnO/SnO₂hierarchical nanostructures could be tailored by changing the growth time of SnO₂nanowires. A possible formation process and growth mechanism are proposed from the viewpoint of inside-out Ostwald ripening. In addition, gas sensors based on the hierarchical ZnO/SnO₂nanostructures were fabricated and exhibited good response to ethanol. At an ethanol concentration of 100 ppm, the response was about 11 and the response time was about 1 s at the operating temperature of 275℃.