The　carbon　content　was　optimized　in　the　synthesis　of　WC-Co　composite　powder　by　in-situ　reactions.　The　particle　sizes　were　optimized　for　the　grain　growth　inhibitors　(GGI)　of　VC　and　Cr3C2.　A　series　of　experiments　were　performed　using　various　GGI　with　different　particle　sizes.　The　effects　of　carbon　content　and　GGI　characteristics　on　the　properties　of　cemented　carbide　bulks　prepared　by　sinter-HIP　were　studied.　A　transverse　rupture　strength　higher　than　5100　MPa　was　achieved　in　the　ultrafine　WC-12Co　cemented　carbide　bulk,　which　had　the　best　combination　of　carbon　content　in　the　in-situ　synthesis　of　composite　powder　and　GGI　particle　size　in　the　sintered　material.　It　was　found　that　with　decreasing　the　Cr3C2　particle　size,　the　toughness　and　strength　of　the　sintered　bulk　materials　increased　significantly　and　reached　a　maximum.　The　opposite　trend　appeared　when　decreasing　the　VC　particle　size.　Moreover,　addition　of　nanoscale　VC　particles　caused　inhomogeneous　microstructure　and　hence　deteriorated　the　fracture　strength.　The　weakening　of　wettability　of　WC　grains　towards　Co　binder　by　the　VC　nanoparticles　was　considered　the　reason.