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 similar to 930 degrees C. 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.