Metal oxide semiconductor (MOS) gas sensors possess extensive applications due to their high sensitivity, low cost, and simplicity. To boost their excellent sensing performance and meet the growing demand for applications, a series of strategies have been developed, such as the surface morphology engineering and function manipulation. Recently, the controlled morphology with exposed high-energy facets and the facet-dependent sensing properties have attracted much attention. Because of its abundant unsaturated active sites, the crystal planes with high surface energy usually serve as promising platform for gas sensing. After a lot of survey of literature, the authors provide a review of recent efforts on engineering crystal structures with exposed high-energy facets of MOS nanomaterials and their improved gas-sensitive performance, emphasis on six kinds of common gas-sensitive MOS including ZnO, SnO₂, TiO₂, α-Fe₂O₃, NiO and Cu₂O. Also, the relationship between dangling bonds density and gas-sensing properties has been systematically discussed and used as one significant factor to evaluate superior sensing surface of MOS. According to the research and calculation, surface engineering by selectively exposing high-energy facets provides an effective way to obtain MOS gas-sensitive materials with superior performance. The understanding of the facet-dependent properties of MOS will assist in and guide the fabrication of more excellent gas sensors in the future.