To　study　the　phase　stability　and　the　phase　transformation　behavior　in　the　nanocrystalline　(NC)　stoichiometric　alloys,　a　thermodynamic　model　has　been　developed　in　the　present　paper.　Using　the　NC　Sm-Co　alloy　as　an　example,　the　thermodynamic　properties　of　various　phases　in　the　alloy　system　were　evaluated　systematically.　In　particular,　the　grain-size-dependence　of　the　Gibbs　free　energy　of　each　alloy　phase　at　different　temperatures　was　provided.　Based　on　the　model　calculations,　the　stabilities　of　different　phases　in　the　NC　Sm-Co　system　were　analyzed.　As　distinctly　different　from　the　phase　stability　in　the　conventional　polycrystalline　alloys,　the　Gibbs　free　energies　of　some　NC　phases　become　positive　at　the　room　temperature　when　the　nanograin　size　is　reduced　to　below　a　certain　critical　value,　which　implies　that　these　phases　cannot　stably　exist　at　the　room　temperature.　In　order　to　verify　the　thermodynamic　model,　the　stoichiometric　Sm-Co　alloy　was　prepared,　and　the　grain　structure　and　the　phase　constitution　of　the　alloy　were　characterized　by　combining　x-ray　diffraction　and　transmission　electron　microscopy　analyses.　The　experimental　findings　have　confirmed　the　thermodynamic　model　predictions　for　the　NC　alloy　system.