This　study　realized　modulation　of　size　and　content　of　metallic　nanoparticles　inside　ceramic　grains　of　cermets　using　WC-Co　cemented　carbides　as　an　example.　The　approach　was　based　on　tailoring　the　composition　of　the　in-situ　reactants　of　synthesizing　ceramic-metal　composite　powders.　With　the　optimized　size　and　content　of　nanoparticles　distributing　in　WC　grains,　simultaneous　improvement　in　hardness,　strength,　and　fracture　toughness　was　achieved　in　the　prepared　WC-Co　cemented　carbides,　and　the　integrated　mechanical　properties　reached　the　highest　among　the　cermets　with　similar　compositions　reported　in　the　literature.　The　effect　of　metallic　nanophases　in　ceramic　grains　on　the　microscale　stress　and　strain　distributions　of　the　cermets　was　quantitatively　investigated.　Detailed　characterization　and　analysis　of　the　interactions　between　the　metallic　nanophases　and　ceramic　dislocations　revealed　that　these　nanophases　hindered　dislocation　motion　but　did　not　cause　local　stress　concentration　and　microcrack　nucleation,　hence　enabled　to　concurrently　strengthen　and　toughen　the　ceramic　grains.　This　study　provides　a　unique　method　and　quantitative　guideline　for　developing　cermets　with　superior　integrated　mechanical　properties,　particularly　important　for　cermet　materials　with　low　metal　contents.　©　2025