In　contrast　to　the　conventional　polycrystalline　low-Co　Sm-Co　alloys　that　have　very　weak　permanent　magnetic　properties,　the　Sm2Co7　alloy　has　been　found　to　have　fairly　promising　permanent　magnetic　performance　when　its　grain　size　is　reduced　to　the　nanoscale.　It　was　discovered　that　the　crystal　structure　of　the　nanocrystalline　Sm2Co7　has　a　strong　nanograin-size-dependent　stability.　The　rhombohedral　structure　of　Sm2Co7　phase　which　is　metastable　at　temperatures　lower　than　1435　K　in　conventional　polycrystalline　system　can　exist　stably　at　room　temperature　in　the　nanocrystalline　system.　To　understand　the　phase　stability　of　the　nanocrystalline　Sm2Co7,　the　experimental　and　nanothermodynamic　analyses　were　combined　to　describe　quantitatively　the　phase　transformation　behavior　of　Sm2Co7　on　the　nanoscale.　The　results　are　important　for　the　development　of　nanostructured　Sm-Co　permanent　magnets.　(C)　2013　Elsevier　B.V.　All　rights　reserved.