Nanocrystalline　alloys　exhibit　apparently　different　phase　transformation　characteristics　in　comparison　to　the　conventional　polycrystalline　alloys.　The　special　phase　stability　and　phase　transformation　behavior,　as　well　as　the　essential　mechanisms　of　the　nanocrystalline　alloys,　were　described　quantitatively　in　a　nanothermodynamic　point　of　view.　By　introducing　the　relationship　between　the　excess　volume　at　the　grain　boundary　and　the　nanograin　size,　the　Gibbs　free　energy　was　determined　distinctly　as　a　function　of　temperature　and　the　nanograin　size.　Accordingly,　the　grain-size-dependence　of　the　phase　stability　and　phase　transformation　characteristics　of　the　nanocrystalline　alloy　were　calculated　systematically,　and　the　correlations　between　the　phase　constitution,　the　phase　transformation　temperature　and　the　critical　nanograin　size　were　predicted.　A　series　of　experiments　was　performed　to　investigate　the　phase　transformations　at　room　temperature　and　high　temperatures　using　the　nanocrystalline　Sm2Co17　alloy　as　an　example.　The　phase　constitution　and　phase　transformation　sequence　found　in　nanocrystalline　Sm2Co17　alloys　with　various　grain-size　levels　agree　well　with　the　calculations　by　the　nano　thermodynamic　model.　(C)　2009　Acta　Materialia　Inc.　Published　by　Elsevier　Ltd.　All　rights　reserved.