The　deformation　characteristics　of　WC,　as　a　typical　hard　ceramic　material,　were　studied　on　the　nanoscale　using　atomistic　simulations　for　both　the　single-crystal　and　polycrystalline　forms　under　uniaxial　compression.　In　particular,　the　effects　of　crystallographic　orientation,　grain　boundary　coordination　and　grain　size　on　the　origin　of　deformation　were　investigated.　The　deformation　behavior　of　the　single-crystal　and　polycrystalline　WC　both　depend　strongly　on　the　orientation　towards　the　loading　direction.　The　grain　boundaries　play　a　significant　role　in　the　deformation　coordination　and　the　potential　high　fracture　toughness　of　the　nanocrystalline　WC.　In　contrast　to　conventional　knowledge　of　ceramics,　maximum　strength　was　obtained　at　a　critical　grain　size　corresponding　to　the　turning　point　from　a　Hall-Petch　to　an　inverse　Hall-Petch　relationship.　For　this　the　mechanism　of　the　combined　effect　of　dislocation　motion　within　grains　and　the　coordination　of　stress　concentration　at　the　grain　boundaries　were　proposed.　The　present　work　has　moved　forward　our　understanding　of　plastic　deformability　and　the　possibility　of　achieving　a　high　strength　of　nanocrystalline　ceramic　materials.