A　nanoscale　thermodynamic　approach　is　developed　to　quantify　the　multiphase　equilibrium　in　nanocrystalline　alloy　systems.　The　nanoscale　multiphase　equilibrium　calculations　indicate　that　the　activities　and　mole　numbers　of　phases　in　alloy　systems　change　diversely　as　a　function　of　the　nanograin　size　at　constant　temperature,　which　results　in　distinctly　different　characteristics　of　phase　stability　and　phase　transformation　behavior　in　nanocrystalline　alloys　with　respect　to　coarse-grained　polycrystalline　counterparts.　Take　the　nanocrystalline　Sm-Co　alloy　system　as　an　example,　the　diverse　phase　stabilities　and　phase　transformations　are　demonstrated.　To　verify　the　thermodynamic　predictions,　a　series　of　experiments　on　preparation　and　characterization　of　nanocrystalline　SmCo3,　Sm2Co7　and　SmCo2　alloys　have　been　performed.　The　experimental　results　are　in　agreement　with　the　model　calculations,　which　validate　the　present　thermodynamic　approach　for　nanoscale　multiphase　equilibrium.