The　characteristics　of　phase　transformation　in　nanocrystalline　alloys　were　studied　both　theoretically　and　experimentally　from　the　viewpoint　of　thermodynamics.　With　a　developed　thermodynamic　model,　the　dependence　of　phase　stability　and　phase　transformation　tendency　oil　the　temperature　and　the　nanogram　size　were　calculated　for　the　nanocrystalline　Sm2Co17　alloy.　It　is　thermodynamically　predicted　that　the　critical　grain　size　for　the　phase　transformation　between　hexagonal　and　rhombohedral　nanocrystalline　Sm2Co17　phases　increases　with　increasing　temperature.　When　the　grain　size　is　reduced　to　below　30　nm,　the　hexagonal　Sm2Co17　phase　can　stay　stable　at　room　temperature,　which　is　a　stable　phase　only　at　temperatures　above　1520　K　in　the　conventional　polycrystalline　alloys.　A　series　of　experiments　were　performed　to　investigate　the　correlation　between　the　phase　constitution　and　the　grain　structure　in　the　nanocrystalline　Sm2Co17　alloy　with　different　grain　size　levels.　The　experimental　results　agree　well　with　the　thermodynamic　predictions　of　the　grain-size　dependence　of　the　room-temperature　phase　stability.　It　is　proposed　that　at　a　given　temperature　the　thermodynamic　properties,　as　well　as　the　phase　stability　and　phase　transformation　behavior　of　the　nanocrystalline　alloys,　are　modulated　by　the　variation　of　nanogram　size,　i.e.　the　grain　size　effects　oil　the　structure　and　energy　state　of　the　nanogram　boundaries.