One　Al　6061　with　gradient　microstructure　is　prepared　by　friction　stir　processing　(FSP).　Microscopic　characterization　indicates　that　the　regulation　of　grain　size　gradient　could　be　achieved　by　adjusting　the　process　parameters　of　FSP.　The　mechanical　property　test　indicates　that　one　gradient　Al　6061　combines　an　ultimate　tensile　strength　of　266　MPa　with　a　uniform　elongation　of　27　%,　and　the　other　comes　258　MPa　with　29　%,　which　provides　an　excellent　combination　of　strength　and　ductility.　Moreover,　the　molecular　dynamics　(MD)　method　is　employed　to　reconstruct　and　simulate　the　mechanical　behavior　of　the　gradient　region　and　the　dynamic　evolution　of　the　microstructure.　When　the　stacking　faults　(SFs)　with　antisymbolic　Burgers　vector　meet　and　detwinning　occurs,　grain　refinement　will　induce　localized　strengthening.　When　the　Shockley　partials　occur　cross-slip,　numerous　sessile　dislocations　obstruct　the　dislocation　motion　more　facilitate　strain　hardening.　Meanwhile,　a　unique　interaction　mechanism　between　shear　bands　(SBs)　and　intergranular　cracks　in　Al　6061　is　discovered　in　plastic　deformation.　The　coarse　net-like　SBs　in　the　small-grain　region　are　highly　susceptible　to　catastrophic　fracture　of　the　material,　and　the　large　cracks　formed　within　the　severely　band-like　SBs　in　the　large-grain　region　weaken　the　strength　substantially.　However,　the　formation　of　sessile　dislocations　in　the　gradient　region　effectively　mitigates　strain　localization　and　inhibits　the　nucleation　of　large　cracks.　The　results　interpret　the　dynamic　evolution　process　of　gradient　microstructure　prepared　by　FSP　and　the　mechanism　of　strengthening　and　toughening　and　provide　process　and　theoretical　references　for　the　research　and　development　of　new　engineering　materials.　©　2024　Elsevier　B.V.