A large quantity of highly uniform SiC nanowires was directly synthesized from Si substrates using a thermal heating method. The atomic structure, morphology, and composition of the SiC nanowires were strongly dependent on the ambient gases (Ar,N-2) used during nanowire-growth. The nanowires grown in Ar were coated with amorphous carbon layers while the nanowires grown in N-2 had no coating layers. Also, nitrogen and oxygen were incorporated only into SiC nanowires grown in N-2. A proposed model for the role of ambient gases in the SiC nanowire growth is discussed. (c) 2005 American Vacuum Society.