Gas-sensing properties of metal oxides in various morphologies has been widely investigated in the past decades. Among the present research, many evidences demonstrate that the morphologies and specific surface area of sensing materials are not the fundamental factors which determine their sensing performance. In this work, ZnO hexagonal plates and ZnO hexagonal prisms which expose high proportion $\left\{0001\right\}$ and $\left\{10\stackrel{-}{1}0\right\}$ crystal facets, respectively, were prepared by hydrothermal method. The effect of exposed crystal facets on the gas-sensing performance of ZnO was studied in detail. The morphology and crystal structure of the materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The gas sensing performance of the composites were also investigated. Results showed that the ZnO with high proportion of exposed $\left\{0001\right\}$ crystal facets exhibited better gas-sensing performance. The analysis and calculation of the two types crystal facets showed that the dangling bonds density of {0001} facets is nearly twice higher than that of $\{10\stackrel{-}{1}0\}$facets, which provides more active sites for gas adsorption and desorption. The results demonstrate that the research on the relationship between exposed crystal facets and gas-sensing performance of metal oxides are of great significance for manufacture of gas sensors.