WC-Co　cemented　carbides　were　prepared　by　liquid-state　sintering　of　in　situ　synthesized　composite　powders　with　a　constant　Co　content　but　different　carbon　concentrations,　and　with　different　size　scales　of　VC　particles　as　grain-growth　inhibitor.　With　an　optimized　carbon　addition　and　doping　with　microscale　VC　particles,　an　ultrahigh　fracture　strength　with　a　mean　value　above　5000　MPa　was　achieved　for　cemented　carbides.　By　detailed　crystallographic　analysis　of　the　configuration　and　interactions　of　the　WC,　Co　and　VC　phases,　the　effects　of　VC　particle　size　on　the　microstructure　and　mechanical　properties　of　cemented　carbides　are　identified.　The　mechanisms　by　which　the　fracture　strength　depends　on　the　VC　particle　size　contain　the　effects　on　the　changes　in　Co　binder　distribution,　atomic　matching　at　the　phase　boundary　and　WC　grain　size.　The　dominant　factors　for　ultrahigh　fracture　strength　of　cemented　carbides　are　proposed.