The　grain　boundary　diffusion　(GBD)　technology　was　used　to　prepare　high　performance　Nd-Fe-B　sintered　magnets　by　NdH3　and　TbH3　nanoparticle　diffusion.　The　factors　affecting　the　coercivity　of　GBD　magnets　include　distribution　of　rare　earth　rich　grain　boundary　phase　and　substitution　of　the　heavy　rare　earth.　In　order　to　distinguish　the　influence　of　various　factors　on　the　coercivity,　the　microstructure　and　magnetic　domain　evolution　of　the　original,　reference,　Nd-diffused,　and　Tb-diffused　magnets　were　analyzed.　The　core-shell　structure　formed　by　heavy　rare　earth　substitution　is　the　main　factor　of　coercivity　enhancement,　and　it　can　transform　the　magnetic　domain　reversal　mode　from　easy-nucleation　(EN)　to　difficultnucleation　(DN).　With　increasing　the　diffusion　depth,　the　shell　of　the　core-shell　structure　gradually　becomes　thinner,　DN　grains　gradually　decrease　while　the　EN　grains　gradually　increase,　indicating　that　the　magnetic　domain　reversal　mode　is　directly　related　to　the　core-shell　structure.　0　2020　Chinese　Society　of　Rare　Earths.　Published　by　Elsevier　B.V.　All　rights　reserved.