The　accurate　quantitative　relationship　between　the　excess　volume　at　the　grain　boundary　and　the　nanograin　size　in　nanocrystalline　alloy　is　deduced.　The　fundamental　thermodynamic　function　of　nanocrystalline　alloy　is　derived　as　a　function　of　nanograin　size　and　temperature.　By　taking　the　SmCo7　alloy　for　example,　the　thermal　stability　of　the　nanocrystalline　alloy,　as　well　as　its　evolution　characteristics,　is　studied　based　on　the　calculated　excess　Gibbs　free　energy　of　nanograin　boundary.　The　results　show　that　the　nanostructure　with　grain　size　below　a　critical　value　that　corresponds　to　the　maximum　excess　Gibbs　free　energy　can　have　higher　thermal　stability　than　a　coarser　nanograin　structure.　Once　the　grain　size　is　larger　than　the　critical　value,　the　nanostructure　may　lose　its　stability　and　undergo　discontinuous　grain　growth.　By　combining　the　nanothermodynamic　model　with　the　cellular　automaton　algorithm,　the　quantitative　and　visual　simulations　of　nanograin　growth　in　nanocrystalline　SmCo7　alloy　are　performed.　The　nanograin　growth　behavior　described　by　the　two　approaches　are　consistent　with　each　other,　which　validates　the　conclusion　of　the　thermal　stability　of　nanocrystalline　alloy,　drawn　from　the　present　thermodynamic　study.