Abstract: The tempering embrittlement of 07Cr12NiMoVNb steel is examined through impact test and electron scanning fractographic analysis.It is revealed that two embrittlement temperature ranges,450～525℃ and 575～625℃ exist in the tempering of the steel.The fracture morphology manifesfs that the fracture mechanism changes from quasi-cleavage plus cleavage to cleavage plus intergranular as the tempering temperature rising and the impact toughness of the samples corresponding to intergranular fracture is the lowest.The concept of toughness-brittleness transition,that is FATT50,is adopted for the estimation of tempering temperature in engineering.