Dense　nanocrystalline　cemented　carbide　bulks　were　prepared　using　a　unique　in　situ　synthesized　WC-Co　composite　powder　with　a　super-ultrafine　nanostructure.　Remarkable　enhancement　in　the　fracture　toughness　(with　high　hardness　being　maintained)　was　obtained　in　the　nanocrystalline　cemented　carbides.　Based　on　detailed　studies　on　the　combination　of　WC　and　Co　phases,　the　WC/Co　orientation　relationship　and　the　atomic　correspondence　at　interfaces,　the　mechanisms　for　high　toughness　in　the　present　nanocrystalline　cemented　carbides　were　demonstrated.　The　study　proposed　that　interfacial　characteristics　play　a　significant　role　in　the　toughness　of　the　nanocrystalline　cemented　carbides,　and　provided　an　effective　approach　to　achieve　superior　combination　properties　of　hardness　and　toughness　in　cermet　materials.　(C)　2012　Acta　Materialia　Inc.　Published　by　Elsevier　Ltd.　All　rights　reserved.