Understanding　the　magnetization　reversal　mechanism　of　a　core-shell　structure　grains　formed　in　the　grain　boundary　diffused　Nd-Fe-B　magnet　can　further　improve　the　utilization　efficiency　of　heavy　rare　earth　resources　to　more　effectively　improve　the　coercivity　of　Nd-Fe-B　sintered　permanent　magnet.　In　this　paper,　magnetic　domain　structure　and　its　dynamic　evolution　of　core-shell　structural　grains　in　grain-boundary　diffused　Nd-Fe-B　magnet　were　in-situ　observed.　Wider　domains　are　found　in　shell　regions　due　to　higher　anisotropy　constant　and　lower　saturation　magnetization.　The　domain　motion　revealed　that　the　demagnetization　process　of　core-shell　structure　has　a　multi-step　nucleation　process.　The　nucleation　of　the　core-shell　structural　grain　firstly　occurs　at　the　core　regions,　while　another　reverse　nucleation　appears　at　the　outmost　portion　in　shell-region　at　a　larger　reverse　field.　The　micromagnetic　simulation　results　confirm　the　possibility　of　the　nucleation　and　reversal　process　observed　in　the　experiment.　Our　results　can　help　understanding　the　demagnetization　process　and　the　coercivity　enhancement　mechanism　of　grain-boundary-diffused　NdFe-B　sintered　magnet.(c)　2022　Published　by　Elsevier　B.V.