The　grain　boundary　(GB)　damage　of　long-term　crept　HR3C　(25Cr–20Ni–Nb–N)　austenitic　steel　with　solid　solution　state　was　investigated　by　nanoindentation　test　accompanied　with　in-situ　electron　back-scattered　diffraction.　The　corresponding　microstructure　was　characterized　by　scanning　electron　microscopy　and　transmission　electron　microscopy.　Results　show　that　the　increase　in　nanoindentation　hardness　at　the　GBs　and　triple　grain　junctions　may　be　related　to　the　dislocation　accumulation　and　carbide　growth　during　the　creep.　Coarsened　M23C6　and　dislocations　piling-up　at　the　GB　accelerate　the　nucleation　and　coalescence　of　creep　cavity　along　the　GB.　The　nanoindentation　hardness　in　grains　varies　with　orientation　of　the　stress　axis.　The　orientation　difference　of　neighbor　grains　may　induce　local　high　geometrically　necessary　dislocation　densities　and　strain　gradients　near　the　GB,　consequently　causing　stress　concentration　and　subsequent　crack　growth　at　specific　GBs.　©　2023,　China　Iron　and　Steel　Research　Institute　Group　Co.,　Ltd.