The　evolutions　of　the　phase　composition　and　the　microstructure,　as　well　as　their　effects　on　the　magnetic　performance,　were　investigated　with　a　series　of　annealing　processes　using　the　prepared　nanocrystalline　single-phase　SmCo7　alloy　as　the　starting　material.　The　SmCo7　(1:7H)　phase　is　discovered　to　have　a　good　single-phase　stability　from　the　room　temperature　up　to　600　degrees　C.　A　discontinuous　nanograin　growth　takes　place　in　nanocrystalline　SmCo7　alloy,　accompanied　with　the　destabilization　of　the　single　1:7H　phase,　resulting　in　the　formation　of　the　Sm2Co17　(2:17R)　and　SmCo5　(1:5H)　phases,　which　exist　in　a　form　of　the　phase-transformation　twins　and　the　particulate　precipitates　respectively.　The　optimum　magnetic　performance　is　obtained　in　the　nanocrystalline　1:7H　single-phase　alloys,　and　the　maximum　intrinsic　coercivity　1164.54　kA/m　and　energy　product　95.65　kJ/m(3)　are　archived　in　SmCo7　alloys　with　the　mean　grain　size　of　33　nm　and　29　nm,　respectively.