The non-equilibrium grain-boundary segregation (NGS) isotherms and its kinetics serve to provide a more complete understanding of inter-granular segregation behavior in relation to mechanical properties, not only for the engineering steels but also for a wide range of structural alloys. The NGS of phosphorus and temper embrittlement dynamics on the same heat treatment condition at the same isothermal holding time in two Cr-Mo steels, 12Cr1MoV and 2.25Cr1Mo, was experimentally studied. The fracture behaviour was also observed by tensile tests in situ in a scanning electron microscope (SEM). Results show that both the concentration of phosphorus atoms in grain boundaries and the degree of embrittlement reaches a maximum at the critical time. It can be satisfactorily elucidated by the temper embrittlement mechanism of NGS caused by cooling from solution temperature to isothermal holding temperature.