Large-scaled MoO2 nanowire arrays have been prepared on silicon substrates by thermal evaporation of Mo in a flow of argon gas without using any catalyst. The nanowires are well aligned and vertically oriented to substrate surface. The cross-section of most of the nanowires has a polygonal shape with the width of about 150 nm, and the typical lengths of the nanowires vary from 2 to 4 mum. The X-ray energy dispersive spectroscopy reveals that the nanowires contained elements of Mo and O. X-ray diffraction analyses and high-resolution transmission electron microscopy study show that the MoO2 nanowires are of monoclinic structure. (C) 2003 Elsevier B.V. All rights reserved.