Uniform and vertical indium-oxide nanotube (IONT) arrays embedded well in n-type InP single crystal have been successfully prepared in situ by porous InP-template-assisted chemical vapor deposition (CVD). This IONT/InP nanostructure reveals high sensitivity to humidity at room temperature, which is ascribed to the ultrahigh surface-to-volume ratio of this nanostructure and the large number of oxygen defected states in IONTs. Such a nanostructure of IONT arrays embedded in a III-V semiconductor substrate could be expected to have potential applications, such as superior gas sensors. This work provides a novel approach for fabricating low-melting metal oxide semiconductor nanotubes.