The　nanoindentation　creep　behaviour　of　crept　HR3C　austenitic　steels　was　investigated　to　study　the　microstructural　evolution　of　the　steel　after　long-term　creep　at　650　°C.　The　corresponding　microstructure　evolution　was　characterised　by　optical　microscopy,　scanning　electron　microscopy,　electron　back-scattered　diffraction　and　transmission　electron　microscopy.　Results　show　that　the　nanoindentation　hardness　and　creep　displacement　decreased　with　increasing　creep　time.　The　dominated　nanoindentation　creep　mechanism　was　most　likely　changed　from　dislocation　slip　to　grain　boundary　(GB)　sliding　induced　by　the　decrease　of　dislocations　density　inside　the　grain　and　coarsening　of　carbides　at　the　GB.　The　coarsening　and　growth　of　the　precipitates　distributed　along　the　GB　weakened　the　pinning　effect　of　the　precipitates　and　accelerated　the　migration　of　the　grain　and　twin　boundaries.　With　prolonged　the　creep　time,　large　numbers　of　cavities　were　formed　at　the　grain　boundaries,　which　might　lead　to　terrible　intergranular　corrosion　and　brittle　cracking　under　the　action　of　stress　and　high　temperature.　©　2021　Informa　UK　Limited,　trading　as　Taylor　&　Francis　Group.