In　the　literature,　a　majority　of　studies　on　thermal　properties　of　nanocrystals　have　taken　the　thermal　features　of　interfaces　as　a　representative　of　those　of　the　whole　nanocrystal.　This　treatment　may　cause　big　errors　in　predicting　some　important　parameters　such　as　the　transition　temperatures　and　critical　sizes　of　phase　for　transformation　in　nanocrystalline　materials,　when　the　grain　size　is　larger　than　several　tens　of　nanometers.　In　the　present　paper,　a　model　to　calculate　the　thermodynamic　functions　of　the　whole　nanocrystal　has　been　developed　with　the　consideration　of　the　effects　of　both　the　interfaces　and　the　crystal　in　the　grain　interior.　The　expressions　of　the　enthalpy,　entropy　and　Gibbs　free　energy,　as　a　functions　of　the　excess　free　volume　at　interfaces,　temperature　and　grain　size,　are　derived　for　single-phase　nanocrystals.　Using　the　Co　nanocrystal　as　an　example,　thermal　features　of　interfaces　and　the　bulk　nanocrystal　have　been　compared　with　the　conventional.　Furthermore,　the　characteristics　of　beta-Co　-＞alpha-Co　phase　transformation　in　nanocrystalline　materials　are　studied,　the　transformation　temperatures　at　different　levels　of　grain　size,　as　well　as　the　critical　grain　sizes　at　different　temperatures,　are　predicted　and　compared　with　experimental　results.