The　hard　WC-Co　composites　usually　have　weak　toughness.　The　present　work　solved　this　issue　by　a　new　strategy　of　fabricating　nanocrystalline　composites,　which　consists　of　concurrent　crystallization,　carbonization,　and　sintering　densification　of　the　amorphous　compound　powder.　The　resultant　nanocrystalline　WC-Co　composite　has　distinctly　larger　proportion　of　interfacial　coherency　at　the　grain-and　phase-boundaries　compared　with　that　in　the　conventional　counterparts.　The　formation　mechanisms　of　the　coherent　WC　grain　boundaries,　Co　twinning　boundaries,　and　step-like　semi-coherent　WC/Co　phase　boundaries　were　demonstrated　on　the　atomic　scale.　Moreover,　the　contributions　of　the　characteristic　interfaces　to　the　resistance　against　intergranular　fracture,　which　dominates　the　failure　of　nanocrystalline　composites　in　the　deformation,　were　illustrated　based　on　the　analyses　of　atomic　configuration　and　crystallographic　relationship　at　interfaces.　The　reasons　were　disclosed　for　the　advanced　comprehensive　properties　with　simultaneously　ultrahigh　hardness　and　fracture　toughness　of　the　prepared　nanocrystalline　composite.　The　present　technology　and　mechanisms　for　properties　enhancement　will　be　applicable　to　a　large　variety　of　composite　materials.