A　new　type　of　grain-interior　planar　defect　in　a　ceramic　phase　in　TiC　doped　cemented　tungsten　carbides　was　discovered.　It　is　unique　in　that　the　monolayers　of　metal　atoms　exist　stably　in　ceramic　grains.　The　planar　defects　were　induced　by　the　ordered　heteroatoms　distributing　on　certain　crystal　planes　of　the　matrix,　which　are　distinct　from　the　known　planar　defects　such　as　phase-,　grain-,　and　twin-boundaries,　stacking　faults,　and　complexions.　Detailed　characterization　on　the　atomic　scale　was　performed　for　the　composition,　structure,　and　crystallography　of　the　planar　defects,　and　their　energy　state　and　stability　were　evaluated　by　modeling.　It　was　found　that　the　Ti　monolayer　assists　nucleation　of　the　new　WC　crystal　along　the　normal　direction　to　its　basal　plane.　Due　to　the　disturbance　of　the　heteroatom　layer,　the　deposition　of　W　and　C　atoms　deviates　from　the　regular　sites　occupied　in　the　perfect　crystal　lattice,　resulting　in　variations　of　the　W–C　arrangement　in　the　grain　structure.　Experiments　confirmed　that　tailoring　the　distribution　density　of　the　planar　defects　could　give　the　best　comprehensive　mechanical　performance　with　simultaneously　outstanding　strength　and　fracture　toughness　in　the　materials　containing　the　grain-interior　planar　defects.　This　study　provides　a　new　strategy　to　greatly　enhance　the　mechanical　properties　of　materials　by　introducing　and　tailoring　planar　defects　in　the　grain　interiors.　©　2023　Central　South　University.