The　ultrafine　WC-eta　composite　powder　was　rapidly　synthesized　by　a　novel　technique　of　in　situ　reactions　using　tungsten　oxide,　cobalt　oxide　and　carbon　black　as　raw　materials.　Under　the　optimized　process　conditions,　the　as-synthesized　composite　powder　has　a　mean　particle　size　of　260　nm,　containing　only　WC　and　eta　phases.　Then　the　composite　powder　was　agglomerated　into　the　thermal　spray　powder　with　high　density　and　good　flowability.　Afterwards,　with　this　powder　as　the　feedstock,　the　ultrafine-structured　WC-based　coating　was　fabricated　by　high-velocity　oxy-fuel　spraying　technique.　The　fabricated　coating　has　an　average　hardness　(HV0.3)　as　high　as　(14　060　+/-　340)　MPa　and　only　a　porosity　of　0.3%.　Results　indicate　that　the　ultrafine-structured　WC-based　coating　exhibits　excellent　corrosion　resistance　to　molten　zinc.　The　corrosion　mechanism　can　be　explained　by　the　slow　dissolving　of　the　coating　matrix　from　the　surface　layer　to　the　inner　area　caused　by　the　diffusion　of　molten　zinc　along　micro-cracks,　which　is　then　followed　by　the　formation　of　penetrative　cracks　and　subsequent　massive　exfoliation　of　coating　materials.