Using　the　unique　in-situ　synthesized　WC-Co　composite　powder　with　super-ultrafine　nanostructure,　the　dense　nanocrystalline　cemented　carbide　bulk　materials　were　prepared.　Detailed　studies　on　the　combination　of　WC　and　Co　phases,　characteristics　of　the　WC/Co　phase　boundaries　and　WC/WC　grain　boundaries,　and　the　factors　that　influence　the　matching　between　crystal　planes　at　the　interface　were　performed.　The　effects　of　the　orientation　relationship　and　the　atomic　correspondence　at　interfaces　on　the　mechanical　properties　of　the　nanocrystalline　cemented　carbide　bulk　materials　were　demonstrated,　and　the　mechanisms　were　proposed.　This　study　indicates　that　the　interfacial　characteristics　play　a　significant　role　in　the　excellent　mechanical　properties　of　nanocrystalline　cemented　carbides,　particularly　the　high　toughness,　and　thus　provides　an　effective　approach　to　achieve　superior　combination　properties　of　hardness　and　toughness　in　nanostructured　cermet　materials.