A　thermodynamic　model　that　describes　the　thermal　properties　of　the　compound　phases　in　the　nanocrystalline　(NC)　alloy　systems　has　been　developed,　which　provides　quantitative　predictions　on　the　stabilities　and　transition　features　of　different　phases.　By　using　the　NC　Sm-Co　alloy　system　as　an　example,　the　temperature　dependence　of　the　mole　Gibbs　free　energy　of　the　NC　alloy　phases　with　different　excess　volumes　in　the　nano-grain　boundary　regions　were　provided.　Based　on　the　model　calculations,　the　relative　stability　of　different　phases　and　the　phase　transition　rules　in　the　NC　Sm-Co　system　were　analyzed.　It　was　shown　by　the　calculation　results　that　near　the　room　temperature,　some　of　the　NC　alloy　phases,　whose　mole　Gibbs　free　energy　values　change　from　negative　to　positive　due　to　the　decrease　of　the　nano-grain　size,　will　transform　into　stable　NC　alloy　phases.　This　is　distinctly　different　from　the　phase　transformation　features　in　the　coarse-grained　alloy　systems,　whose　thermodynamic　properties　are　only　dependent　on　the　temperature.　The　experimental　results　on　the　phase　configuration　and　the　phase　stability　obtained　from　the　prepared　NC　Sm-Co　alloy　confirmed　the　theoretical　predictions　of　the　present　thermodynamic　model.