Nanostructured sub-stoichiometric transition metal oxides hold promising applications in many areas, while challenge still remains towards their precisely controlled synthesis, particularly for those ultrafine nanowires. In present work, a carbon nanotube (CNT) inner growth based on two-step synthetic approach was adopted to achieve one-dimensional sub-stoichiometric tungsten oxides. Ammonium tetrathiotungsten((NH₄)₂WS₄)) were encapsulated into the inner cavity of carbon nanotubes, which were thermally decomposed into tungsten oxides inside the CNTs. Characterizations confirm that the CNT-filling products are ultrafine one-dimensional W₃O₈ nanowires, that appears 1.23-1.93 nm in width (corresponding to four-five arrays of tungsten atomic column) and up tens of microns in length (close to that of CNTs). The non-noticeable size-dependent lattice parameters of the W₃O₈ nanowires and the large nanowire-CNT separation indicate a rather weak interaction between the filled W₃O₈ nanowires and host CNTs, which provides the opportunity to separate and probe the intrinsic properties of these ultrafine 1D W₃O₈ nanowires. Nevertheless, the proposed synthetic method is extendable to achieve other 1D transition metal oxides.