Single crystalline K 0.4 WO 3 whiskers were successfully prepared at temperature of 360°C through a hydrothermal reaction using WO 3 as tungsten source, KOH as potassium source, and hydrazine hydrate (N 2 H 4 ·H 2 O) as reducing agent. X-ray powder diffraction showed the as-prepared whiskers was tetragonal K 0.475 WO 3 with cell parameters of a=12.29Å and c=3.824Å. Scanning electron microscope observation showed that the size of whiskers reached up to 2mm in length and 6–25μm in diameter. The influence of some reaction parameters, including the pressure-relief process, on the growth of the whiskers was studied. The possible chemical solution transport mechanism was also discussed.