Tungsten oxide (WOx) nanostructures were prepared by a hot filament chemical vapor deposition (HFCVD) system. Two series of samples were synthesized by adjusting filaments temperature (Tf) and oxygen gas concentration (OGC). The crystallinity and stoichiometry were highly related to Tf and OGC. The evolution of stoichiometry and types of defects was illuminated by slow positron implantation spectroscopy (SPIS). A turning point of Tf=1750°C was found that at this point the crystallinity and stoichiometry natures were the best. In order to develop the chemical phase from substoichiometric to stoichiometric, the oxygen gas concentration in the mixture gas during deposition should be raised to an appropriate level.