Abstract: Effects of cryogenic treatment temperatures and times at the temperature on microstructure and properties of M35 high speed steel were investigated by X-ray diffractometer,scanning electron microscope,universal tensile tester,Rockwell hardness tester and impact tester.The results showed that（a） the cryogenic treatment caused primary carbides to reduce in amount and secondary carbides to precipitate dispersively in the M35 steel;（b） with the decrease in cryogenic treatment temperature,the carbides exhibited a more uniform distribution and an increase in size;（c） with the extension of cryogenic treatment time,the secondary carbides increased in both amount and aggregation degree;（d） the temperature and time of cryogenic treatment had less effect on the hardness and more significant effect on both impact toughness and bending strength for the steel after tempering;（e） the lower the cryogenic treatment temperature,the higher the impact toughness and bending strength were,the impact toughness and bending strength of the steel cryogenically treated at -190 ℃ for 2 h were 62. 3 J/cm² and 4 950 MPa,respectively;（f） with the extension of cryogenic treatment time,the impact toughness of the steel increased at first and then decreased,and its the bending strength increased gradually,the steel cryogenically treated at 90 ℃ for 6 h exhibited impact toughness as the highest as 57. 3 J/cm²,and the steel cryogenically treated at -90 ℃ for 10 h exhibited bending strength as the largest as 4 810 MPa.