The　microscopic　structure　change　rule　in　high　speed　steel　(HSS)　after　an　additional　cryogenic　treatment　was　studied.　Highly　dispersed　nano-carbide　(η-Fe2C)　separated　from　tempering　martensite　was　found　distributed　on　twin　planes　of　martensite　and　dislocation　places.　Retained　austenite　was　found　sheared　into　martensite　and　nano-carbide　separated　at　the　same　time.　In　addition,　the　tempering　martensite　was　fine-grained.　The　stated　changes　of　HSS　evidently　improve　the　lifespan　of　HSS　cutter　and　fully　show　the　advantages　of　nano-structure.　The　cryogenic　treatment　of　HSS　also　improves　the　roughness　of　HSS　cutter　(decrease　Ra　number　by　about　50%)　and　greatly　increases　the　wear　resistance　of　HSS　cutter.　Further　experiments　on　the　wear　characteristic　of　HSS　cutter　supported　the　findings　mentioned　above.　The　stated　HSS　treatment　technology　would　implement　nano-structure　of　traditional　tool　steel　through　phase　change.　It　could　be　a　new　method　to　improve　the　performance　of　HSS　cutter.