High-purity Si nanowires in bulk-quantity have been grown from SiO in a high-temperature tube furnace. Thermal sublimation of SiO powders produced SiO vapor, which was transported and deposited on the inner wall of the tube at ∼930oC. We proposed that the SiO deposit underwent a disproportionation reaction to form nanowires containing Si and SiO2. Each wire consisted of a single crystalline Si core and an oxide sheath, 6–28 nm in diameter and up to 1 mm in length. This work gave strong support to the recently proposed oxide-assisted growth mechanism. The yield of Si nanowires increased with sublimation temperature and pressure. Under suitable deposition conditions, Si nanowire growth rate of more than 10 mg/h could be routinely obtained.