Quasi-aligned W18O49 nanotubes and nanowires with single-crystalline monoclinic structure were prepared via a physical vapor deposition process. The axes of the nanotubes and nanowires were aligned along the [010] direction. Field-emission measurement revealed a threshold field of 6.2 V mum(-1) for W18O49 nanowire films, indicating the prepared W18O49 nanowires are excellent field emitters.