We report a successful fabrication of a thin nanosheet assembled flower-like NiO–ZnO composite with PN heterojunction for gas-sensing application. We find that the obtained novel composites show excellent sensing performances, which are attributed to the depletion layer induced by the existence of PN heterojunction and also to the establishment of potential barriers. Moreover, the thin thickness of the nanosheet in comparison to the electron depletion layer is also found to account for the improved gas-sensing properties. The findings demonstrate advance of the hierarchical nanostructures in further enhancing gas-sensing properties, and such facile method could be applied to produce many other gas-sensing materials.