A　phase-field　model　is　presented　which　considers　the　accumulation　of　structural　defects　in　grain　boundaries　by　an　isotropic　eigenstrain　associated　with　the　grain　boundaries.　It　is　demonstrated　that　the　elastic　energy　caused　by　dilatation　of　the　grain　boundary　with　respect　to　the　bulk　crystal　contributes　largely　to　the　grain　boundary　energy.　The　sign　of　this　contribution　can　be　both　positive　and　negative　dependent　on　the　local　stress　state　in　the　grain　boundary.　Self-diffusion　of　atoms　is　taken　into　account　to　relax　the　stress　caused　by　the　dilatation　of　the　grain　boundary.　Application　of　the　model　to　discontinuous　grain　growth　in　pure　nanocrystalline　cobalt　material　is　presented.　Linear　grain　growth　is　found　in　the　nanocrystalline　state,　which　is　explained　by　the　interpretation　of　grain　boundary　motion　as　a　diffusive　process　defining　an　upper　limit　of　the　grain　boundary　velocity　independent　of　the　grain　boundary　curvature　but　dependent　on　temperature.　The　transition　to　regular　grain　growth　at　a　critical　temperature,　as　observed　experimentally,　is　explained　by　the　drop　of　theoretical　grain　boundary　velocity　due　to　its　mean　curvature　during　coarsening　of　the　nanograin　structure　below　the　maximum　velocity.