Single crystalline K0.4WO3 whiskers were successfully prepared at temperature of 360degreesC through a hydrothermal reaction using WO3 as tungsten source, KOH as potassium source, and hydrazine hydrate (N2H4 . H2O) as reducing agent. X-ray powder diffraction showed the as-prepared whiskers was tetragonal K0.475WO3 with cell parameters of a = 12.29 Angstrom and c = 3.824 Angstrom. Scanning electron microscope observation showed that the size of whiskers reached up to 2 mm in length and 6-25 mum 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. (C) 2002 Elsevier Science B.V. All rights reserved.