A　thermodynamic　model,　into　which　the　nanoscale　effect　is　introduced,　is　established　to　describe　the　thermal　parameters　of　metastable　phase,　as　well　as　their　changes　with　the　concentration,　temperature　and　grain　size.　Based　on　the　model　calculations　and　experiments,　the　thermodynamic　properties　and　dependence　on　the　grain　size　are　disclosed.　Taking　the　metastable　SmCo7　phase　for　example,　the　conditions　for　the　single-phase　stability　and　the　phase　decomposition　are　studied.　The　results　are　important　for　the　control　of　the　phase　stability　and　phase　transformation　of　the　metastable-phase　alloy.