Current　WC-Co　cemented　carbide　coatings　are　mainly　prepared　by　micro-size　powders.　In　the　present　work,　the　ultrafine　WC-Co　composite　powder　was　rapidly　synthesized　by　the　technique　of　in　situ　reactions.　By　controlling　the　carbon　addition　of　the　raw　powders　as　17.04%　in　mass　fraction,　the　as-synthesized　ultrafine　WC-12Co　composite　powder　only　contains　WC　and　Co　phases,　with　an　average　particle　size　of　210　nm.　The　ultrafine　structured　WC-12Co　cemented　carbide　coating　was　fabricated　by　the　high　velocity　oxy-fuel　method,　using　the　agglomerated　and　sintered　composite　powder　particles.　The　results　indicate　that　the　coating　fabricated　by　the　ultrafine　composite　powder　has　obviously　lower　porosity,　30%　higher　hardness　as　up　to　1300HV0.3,　and　better　wear　resistance　especially　in　larger　load　than　those　of　the　coating　fabricated　by　conventional　commercial　powder.　The　results　demonstrate　that　the　ultrafine　WC-Co　composite　powder　has　superior　advantages　in　preparing　cemented　carbide　coatings　with　high　performance.