The　thermodynamics　of　mechanisms　of　the　reactions　in　the　synthesis　processes　of　WC-Co　composite　powder　by　WO3,　CO3O4　and　carbon　as　original　reactants　was　investigated.　By　the　thermodynamic　calculations,　the　initiation　of　the　in　situ　reduction　and　carbonization　reacfions,　the　formation　sequence　and　the　relative　stability　of　the　reaction　products,　can　be　quantitatively　described.　The　formation　sequence　of　the　reaction　products,　and　the　fact　that　WC-Co　composite　powder　with　pure　phases,　homogeneous　and　ultrafine　particles　can　be　synthesized　at　1323　K　in　the　vacuum　condition,　as　found　in　experiments,　verified　the　thermodynamic　predictions.　The　thermodynamic　analysis　performed　in　the　present　work　is　significant　to　control　the　preparation　processes　and　to　optimize　the　parameters　of　fast　synthesizing　pure-phased　WC-Co　composite　powder.　(C)　2007　Elsevier　B.V.　All　rights　reserved.