The　coercivity,　microstructure,　and　magnetic　domain　structure　of　Nd-Fe-B　sintered　magnets　by　grain　boundary　diffusion　process　(GBDP)　with　TbH3　nanoparticles　were　systematically　investigated.　Compared　to　the　original　magnet,　the　coercivity　(H-ci)　of　the　GBDP　magnets　improved　from　1702　to　2374　kA　center　dot　m(-1)　with　few　remanence　reduced　from　1.338　to　1.281　T.　Electron　probe　micro-analysis　(EPMA)　analysis　showed　that　Tb　diffused　along　grain　boundary,　mainly　concentrated　in　the　boundary　layer　of　the　main　phase,　and　formed　a　core-shell　structure.　Magneto-optical　Kerr　optical　microscope　(MOKE)　analysis　showed　that　there　were　two　types　of　magnetic　domain　reversal　in　one　grain:　gradual　reversal　(GR)　and　abrupt　reversal　(AR).　When　the　applied　field　decreased　from　saturated　magnetic　field,　the　reversal　magnetic　domain　nucleated　and　then　spread　over　the　whole　grain　gradually,　which　was　called　GR.　However,　some　grains　kept　the　single　domain　state　until　H-h　which　was　a　value　of　reverse　direction　applied　field　in　second　quadrant　in　hysteresis　loops.　When　the　applied　field　increased　above　H-h,　reversed　magnetic　domain　would　suddenly　appear　and　occupy　most　of　the　area　of　the　grain,　which　was　called　AR.　That　is　because　AR　grains　have　higher　reversed　magnetic　domain　nucleation　field　(H-RN2)　than　GR　grains　(H-RN1).　After　GBDP,　the　area　of　AR　region　increased　obviously　and　GR　region　decreased　accordingly,　indicating　that　the　core-shell　structure　could　change　GR　grain　into　AR　grain.　The　core-shell　structure　could　suppress　flipping　of　the　magnetization　of　the　grains　due　to　the　large　magnetic　anisotropy　of　Tb-rich　shell.　Therefore,　large　AR　area　led　to　high　coercivity.