Magnetoelasticity　is　the　bond　between　magnetism　and　mechanics,　but　the　intricate　mechanisms　via　which　magnetic　states　change　due　to　mechanical　strain　remain　poorly　understood.　Here,　we　provide　direct　nanoscale　observations　of　how　tensile　strain　modifies　magnetic　domains　in　a　ferromagnetic　Ni　thin　plate　using　in　situ　Fresnel　defocus　imaging,　off-axis　electron　holography　and　a　bimetallic　deformation　device.　We　present　quantitative　measurements　of　magnetic　domain　wall　structure　and　its　transformations　as　a　function　of　strain.　We　observe　the　formation　and　dissociation　of　strain-induced　periodic　180　&　DEG;　magnetic　domain　walls　perpendicular　to　the　strain　axis.　The　magnetization　transformation　exhibits　stress-determined　directional　sensitivity　and　is　reversible　and　tunable　through　the　size　of　the　nanostructure.　In　this　work,　we　provide　direct　evidence　for　expressive　and　deterministic　magnetic　hardening　in　ferromagnetic　nanostructures,　while　our　experimental　approach　allows　quantifiable　local　measurements　of　strain-induced　changes　in　the　magnetic　states　of　nanomaterials.　When　strain　is　applied　to　some　magnetic　materials,　the　magnetic　properties　and　magnetization　can　change　drastically.　This　coupling　is　referred　to　as　magnetoelasticity,　and　while　its　history　of　study　is　long,　it　is　still　not　a　well-understood　phenomenon.　In　this　work,　Kong　et　al.　shed　light　on　magnetoelasticity　using　a　variety　of　experimental　probes.