Dense　ultrafine-structured　cermet　coatings　with　low　decarburization　were　prepared　using　the　in　situ　synthesized　WC-Co　composite　powder,　which　had　been　subjected　to　agglomeration　and　heat-treatment　prior　to　thermal　spraying.　Remarkable　enhancement　in　the　fracture　toughness　was　obtained　in　the　ultrafine-structured　WC-Co　coating.　Detailed　characterization　and　analysis　were　performed　on　the　coating　microstructure　and　the　crack　growth　path　by　SEM　and　TEM　examinations.　The　present　study　demonstrated　that　the　high　density,　the　large　bond　strength　between　splat　boundaries　and　the　increased　contact　area　between　carbides　and　Co　binder　phase　as　compared　to　the　coarser-grained　WC-Co　coatings,　contributed　significantly　to　the　high　toughness　of　the　ultrafine-structured　coating.　Thus　our　work　provides　some　new　understanding　of　the　relationship　between　microstructures　and　properties　of　the　ultrafine-structured　cermet　coatings.