Abstract: Through testing,SEM and TEM observation and XRD analysis for common mechanical properties of 9NiCrMo steel under different tempering temperature,the effect of tempering temperature on mechanical properties and microstructures of the steel was studied. The steel shows relatively high strength and good lowtemperature toughness. As the tempering temperature increases,the yield strength and tensile strength peak near 550℃ and 570℃ respectively. Fine M₂C carbides precipitated play an important role in strengthening. Secondary martensitic precipitation is also a factor that cause the increase in strength. Good lowtemperature toughness can mainly be attributed to the appearance of very tiny secondary martensite lath,ferrite,and appropriate amount of reverted austenite.