The　fundamental　thermodynamic　functions　of　enthalpy,　entropy,　and　Gibbs　free　energy,　as　functions　of　the　excess　free　volume　at　interfaces,　temperature,　and　grain　size,　have　been　derived　for　single-phase　metal　nanocrystals.　The　model　was　applied　to　predict　the　thermal　features　of　nano-gain　boundaries　and　the　characteristics　of　phase　transformation　in　nanocrystalline　metals,　such　as　the　transformation　temperature　and　the　critical　grain　size　for　phase　transformation　at　a　given　temperature.　The　model　predictions　have　been　verified　by　experimental　studies　on　the　beta-Co　＜-＞　alpha-Co　phase　transformation　in　nanocrystalline　Co　prepared　by　ball　milling.