A　non-equilibrium　phase　of　SmCo9.8　that　does　not　exist　in　the　conventional　polycrystalline　Sm-Co　alloy　system　has　been　obtained　in　an　ultrafine　nanocrystalline　alloy　prepared　by　a　developed　method.　The　conditions　for　the　formation　and　stability　of　the　SmCo9.8　phase　were　studied.　It　was　found　that　the　single-phase　SmCo9.8　exists　stably　only　in　nanocrystalline　alloy　with　the　grain　size　smaller　than　10　nm.　With　increasing　the　temperature　and　hence　the　coarsening　of　the　grain　structure,　the　destabilization　of　SmCo9.8　phase　occurs,　leading　to　a　phase　constitution　of　the　Sm2Co17　and　Co　phases.　The　effects　of　the　nanograin　structure　and　the　phase　constitution　on　the　magnetic　performance　of　the　nominal　SmCo9.8　alloys　were　demonstrated.　Based　on　the　preparation　technique　and　detailed　characterizations,　the　present　study　provides　a　new　understanding　of　the　stability　of　the　high-Co　Sm-Co　phase　and　its　evolution　behavior,　which　may　facilitate　the　alloys　design　according　to　the　correlation　of　phase　stability　and　magnetic　performance.　(C)　2012　Elsevier　B.V.　All　rights　reserved.