Gradient　nanostructured　metals　attract　extensive　attention　due　to　their　excellent　mechanical　properties,　while　they　are　prone　to　microcracks　during　forming　and　servicing　process　and　eventually　develop　into　a　sudden　fracture.　Actually,　experimental　studies　show　that　microcracks　occurred　in　both　the　grain　boundary　(GB)　and　the　intragrain,　but　most　of　the　current　reports　only　focus　on　the　GB　cracking,　very　limited　work　has　studied　the　intragranular　cracking.　In　this　paper,　the　crystal　plasticity　finite　element　method　(CPFEM)　and　cohesive　zone　model　(CZM)　are　combined　to　study　the　cracking　mechanism　of　polycrystalline　aluminum　(Al)　with　different　grain-gradient　structures　under　tensile　load,　where　molecular　dynamics　(MD)　method　is　used　to　determine　the　cohesive　parameters　of　the　intragrain　and　GB.　The　results　not　only　show　the　crack　initiation　and　propagation　process　of　polycrystalline　Al　with　different　grain-gradient　structures,　but　also　reveal　the　mechanism　of　intragrain　fracture,　GB　fracture　and　crack　transgranular.　The　grain-gradient　distribution　with　optimal　comprehensive　performance　is　obtained.　Moreover,　it　is　also　found　that　the　initial　microcracks　with　different　positions,　numbers　and　angles　have　a　great　influence　on　the　cracking　mechanism　and　effective　properties　of　the　whole　material.　This　study　provides　a　solid　theoretical　basis　for　improving　the　quality　and　operation　life　of　metal　parts.　©　2020　Elsevier　Ltd