The　thermal　features　of　the　nanograin　boundary　were　described　by　a　developed　thermodynamic　model.　Using　the　nanocrystalline　Cu　as　an　example,　the　pressure,　the　bulk　modulus,　and　the　volume　thermal　expansion　coefficient　were　calculated　to　characterize　the　thermodynamic　properties　of　the　grain　boundaries　on　the　nanoscale.　Based　on　the　parabola-type　relationship　between　the　excess　free　energy　and　the　excess　volume　of　the　nanograin　boundary,　the　thermal　stability,　as　well　as　its　evolution　characteristics,　was　analyzed.　The　experimental　results　of　the　temperature-varying　grain　growth　in　the　nanocrystalline　Cu,　which　exhibited　the　discontinuous　nanograin　growth　behavior,　verified　the　thermodynamic　predictions.　In　addition,　the　quantitative　relationships　correlating　the　excess　volume　and　the　lattice　expansion　with　the　nanograin　size　were　discussed.