The　chemical　reaction　thermodynamics　and　a　novel　two-stage　growth　mechanism　of　SiC　nanowires　synthesized　by　carbothermal　reduction　reactions　were　investigated　based　on　the　Si-C-O　systems　over　a　wide　temperature　range　(1050　＜=　T　＜=　2000　K).　The　carbothermal　reduction　reaction　process　involves　the　fast　formation　of　gaseous　SiO　and　CO　crucial　intermediates,　and　the　further　carbon　reduction　of　SiO　to　SiC.　The　relationship　between　the　free　energy　changes　and　temperature　at　different　pressures　was　also　discussed.　Some　fundamental　data　in　the　work　can　help　to　analyze　the　thermochemistry　of　the　carbothermal　reduction　reaction　in　the　Si-C-O　system,　which　is　beneficial　to　optimize　the　temperature,　pressure　and　the　input　precursors　for　controlling　the　SiC　nanowire　growth.