A new interpretation of temper embrittlement dynamics is proposed, which is based on the diffusion of phosphor atoms to grain boundaries by the complex of phosphor atom-vacancy. The dynamics of temper embrittlement in a medium-carbon Cr steel during 538 degreesC tempering was carefully examined. The results show that the dependence of 50% fracture appearance transition temperature (FATT) on tempering time has a maximum, which can be satisfactorily elucidated by diffusion of the complex of phosphor atom-vacancy. However, the dependence of hardness on tempering duration decreases all the time. The fracture morphology was observed by scanning electron microscopy (SEM), the variation of intergranular fracture ratio also has a maximum during tempering treatment at 538 degreesC, which is at the near same time as the one in the dynamic of temper embrittlement. The concentration of phosphor on grain boundary was measured by Auger electron microscopy (AES).