Abstract: The mechanical properties at low temperatures (room temperature--150 ℃) of the 40CrNi2Mo steel at quenched and tempered state were investigated by using low temperature tensile tests and series impact tests.The results indicate that the yield strength and tensile strength of the steel elevated with the reducing temperature,while the reduction of area and elongation reduced,the above properties and temperature had a linear relationship.The formula of the yield strength and the tensile strength as well as the temperature was obtained for the 40CrNi2Mo steel. According to the drawn impact energy curve,comprehensive energy method and the fracture surface morphology,its ductile-brittle transition temperature could be determined,which was -125 ℃.