Abstract: The study of thermal ageing embrittlement of temperature sets of reactor pressure vessel surveillance Ni-Cr-Mo-V steel weld metal was conducted. Weld metal is the weakness of the reactor pressure vessel and its service time gets to 120 430 h (the operating temperature is normalized to 300 ℃). The impact test of the 3 batches of weld surveillance sample indicates that the weld metal is embrittled during thermal ageing. Through the study, significant change is not found in the microstructure and micro-Vickers hardness during thermal ageing, indicating that there is no hardening embrittlement mechanism in the thermal ageing process. The study of impact fracture and Auger electron spectroscopy indicates that the impurity element P segregates to the grain boundaries and lowers their cohesion during the long-term thermal ageing. Therefore, the reactor pressure vessel Ni-Cr-Mo-V steel weld metal occurs non hardening embrittlement which is probably caused by the intergranular segregation of impurity element P during the long-term thermal ageing.