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A nanoscale thermodynamic model was developed to describe the phase stability and the phase transition features in nanocrystalline(NC)alloys．Using NC Sm-Co alloy aS an example，the thermodynamic properties of different stoichiometric alloy phases were systematically calculated．Particularly,the temperature-dependence of the Gibbs free energy of each alloy phase with different nano-grain sizes were provided．Based on the model calculations，the relative stabilities of different phases in NC Sm-Co system were analyzed．It WaS shown by the calculation results that at the room temperature，when the nano-grain size is reduced to below certain critical values，the Gibbs free energy of SmC05，SmC03 and SmC02 will become positive，which implies that these phases cannot stably exist at room temperature,which is distinctly different from the phase stability in conventional coarse-grained polycrystalline alloys．The experimental studies performed in our group,which WaS focused ON preparation and characterization of stoichiometric NC Sm-Co alloys，confirmed the present nanoscale thermodynamic predictions．