Using　the　nanoscale　violet　tungsten　oxide　as　the　tungsten　source,　the　WC-Co　composite　powder　was　synthesized　by　the　in　situ　reactions.　The　particle　size　of　the　WC-Co　composite　powder　has　a　narrow　distribution　with　the　mean　particle　size　below　100　nm,　and　the　single　composite　particle　has　a　nanocrystalline　structure　with　a　mean　grain　size　smaller　than　10　nm.　The　detailed　characterizations　of　the　nanoparticle　microstructure　reveal　that　the　orientation　relationship　and　coherence　at　the　interfaces　can　form　during　the　in　situ　reactions　and　further　inherit　in　the　consolidated　cemented　carbide　bulk　material.　The　favorable　crystallographic　characteristics　of　the　WC-Co　composite　nanoparticles　play　a　significant　role　in　the　enhancement　of　the　mechanical　properties　of　the　prepared　cemented　carbide　bulk　material.