Sm-Co　alloys　with　the　stabilized　SmCo7　phase　are　most　prominent　candidates　for　advanced　high　temperature　permanent　magnets,　where　the　stabilization　of　the　SmCo7　phase　can　be　effectuated　by　nanostructuring.　The　complex　concurrent　processes　of　ordering　and　phase　transformation　in　a　SmCo7　nanograin　are　characterized　on　the　atomic　scale.　For　the　first　time　early　stages　of　the　phase　transformation　are　made　visible　by　highlighting　specific　superstructures　in　single　nanograins　using　Fourier　reconstruction　of　high-resolution　transmission　electron　microscopy　images.　The　superstructures　are　only　detectable　and　can　only　be　distinguished　in　specific　crystallographic　orientations.　The　evolution　of　the　atom　arrangement　in　the　crystal　structures　is　demonstrated　for　the　concurrent　ordering　process　and　phase　transformation.　During　decomposition　of　the　metastable　SmCo7　phase,　the　hexagonal　Sm2Co17　superstructure　(2:17H)　forms　at　first　as　a　precursor　of　the　rhombohedral　Sm2Co17　superstructure　(2:17R)　-　this　can　only　be　detected　by　analysis　of　individual　grains　and　has　not　been　described　so　far.　By　extensive　crystallographic　analysis　of　individual　nanograins,　a　distinct　correlation　between　the　fraction　of　the　superstructure　phases　and　the　grain　size　is　found,　showing　directly　and　unambiguously　the　grain　size　dependence　of　the　phase　transformation　in　the　nanocrystalline　alloy,　a　phenomenon　that　so　far　has　only　been　shown　indirectly　using　volume　averaging　methods.