In　order　to　clarify　the　basic　mechanical　behavior　of　grain　boundaries　(GBs)　in　Al　under　tensile　stresses,　the　first-principles　tensile　tests　of　coincidence　tilt　and　twist　GBs　in　Al　have　been　performed　by　using　the　projector　augmented　wave　(PAW)　method　based　on　the　density-functional　theory　(DFT)　within　the　generalized　gradient　approximation　(GGA).　For　the　{221}　Sigma　=　9　tilt　and　{001}　Sigma　=　5　twist　GBs　in　Al,　we　have　obtained　ideal　strength　and　stress　strain　curves,　and　analyzed　bond-breaking　behavior.　Results　for　the　Sigma　=　9　tilt　GB　are　consistent　with　previous　results　using　the　local　density　approximation　(LDA).　The　Sigma　=　5　twist　GB　reveals　complex　behavior　of　deformation　and　failure,　different　from　tilt　GBs.　In　both　the　interfaces,　reconstructed　interfacial　bonds　reveal　high　strength,　inducing　large　deformation　in　the　GB　regions　before　the　failure,　due　to　the　covalent　bonding　nature　of　Al.　[doi:10.2320/matertrans.MD201130]