Hydrothermal method with silicon monoxide as starting material was used to synthesize silicon nanowires (SiNWs). Transmission electron spectroscopy dernonstrates that SiNWs have smooth surface and about 35nm in diameter, micrometers in length. High-resolution transmission electron spectroscopy shows that each nanowire consists a polycrystalline Si core and an amorphous silica sheath. Si and silicon oxide are proposed to generate from SiO under the high temperature and high pressure of hydrothermal conditions. The growth mechanism of SiNWs is proposed as the oxide-assisted growth mechanism. (c) 2005 Elsevier B.V. All rights reserved.