We present a novel method to synthesize high-density silicon nitride nanowires directly from the silicon substrates via a catalytic reaction under ammonia or hydrogen flow at 1200 degreesC. Gallium, gallium nitride, and iron nanoparticles deposited on the silicon substrate were used as catalysts. Gallium nitride can act as a nitrogen source under hydrogen flow. The average diameter of the nanowires is 40 nm and their length is about 300 mum. The silicon nitride nanowires consist of a defect-free single-crystalline alpha-phase crystal grown with various growth directions. (C) 2003 Elsevier Science B.V. All rights reserved.