The effects of the surface and orientation of a WO_3 nanowire on the electronic structure are investigated by using first principles calculation based on density functional theory(DFT).The surface of the WO3 nanowire was terminated by a bare or hydrogenated oxygen monolayer or bare WO_2 plane,and the[010]- and[001]-oriented nanowires with different sizes were introduced into the theoretical calculation to further study the dependence of electronic band structure on the wire size and orientation.The calculated results reveal that the surface structure, wire size and orientation have significant effects on the electronic band structure,bandgap,and density of states (DOS) of the WO_3 nanowire.The optimized WO_3 nanowire with different surface structures showed a markedly dissimilar band structure due to the different electronic states near the Fermi level,and the O-terminated[001] WO_3 nanowire with hydrogenation can exhibit a reasonable indirect bandgap of 2.340 eV due to the quantum confinement effect,which is 0.257 eV wider than bulk WO_3.Besides,the bandgap change is also related to the orientation-resulted surface reconstructed structure as well as wire size. The effects of the surface and orientation of a WO_3 nanowire on the electronic structure are investigated by using first principles calculation based on density functional theory (DFT). The surface of the WO3 nanowire was terminated by a bare or hydrogenated oxygen monolayer or bare WO_2 plane , and [010] - and [001] -oriented nanowires with different sizes were introduced into the theoretical calculation to further study the dependence of electronic band structure on the wire size and orientation. calculated results reveal that the surface structure, wire size and orientation have significant effects on the electronic band structure, bandgap, and density of states (DOS) of the WO_3 nanowire. the optimized WO_3 nanowire with different surface structures showed a markedly dissimilar band structure due to the different electronic states near the Fermi level, and the O-terminated [001] WO_3 nanowire with hydrogenation can exhibit a reasonable indirect band gap of 2.340 eV due to the quantum con finement effect, which is 0.257 eV wider than bulk WO_3.Besides, the band gap change is also related to the orientation-induced surface reconstructed structure as well as wire size.