Nature　of　grain　boundaries　(GBs)　should　affect　the　micro-structural　evolution　and　mechanical　properties　of　metallic　micro-crystalline　formed　by　severe　plastic　deformation.　The　stability　and　interfacial　bonding　of　coincidence　tilt　and　twist　GBs　in　At　and　Cu　have　been　examined　by　using　the　projector-augmented　wave　method　within　the　density-functional　theory.　For　the　[221}　Sigma　=　9　tilt　boundary,　glide　models　are　more　stable　than　mirror　models　for　At　and　Cu,　and　the　{001}　Sigma　=　5　twist　boundary　is　more　stable　than　the　Sigma　=　9　tilt　boundary　for　At　and　Cu,　due　to　smaller　structural　distortions.　There　is　a　tendency　that　the　boundary　energies　in　At　are　substantially　smaller　than　those　in　Cu.　This　can　be　explained　by　the　electronic　and　atomic　behavior　of　bond　reconstruction　at　the　interfaces　in　At,　due　to　the　covalent　nature　of　At　as　observed　in　the　charge　density　distribution,　in　contrast　to　rather　simple　metallic　bonding　at　Cu　GBs.　[doi:　10.2320/matertrans.MD200820]