The effect of bulk P contents on hardening, non-equilibrium intergranular segregation and embrittlement has been studied in Mn-doped ferritic alloys subjected to neutron irradiation (E>0.1MeV: fluence of 1 × 1025 n/m2 at 711K for 2120 h) or irradiation-equivalent thermal aging. Neutron irradiation-induced intergranular P segregation became more prominent with decreasing bulk P content. Thermal aging slightly enhanced the amount of segregated P independent of the bulk P content. Intergranular C segregation in all the alloys was suppressed by the irradiation. An alloy with low bulk P content showed only moderate irradiation-induced hardening. The ductile-brittle transition temperature (DBTT) in alloys with low and intermediate amounts of P increased by the same shift during the irradiation but not at all during the thermal aging. Doping high bulk P led to a high DBTT in the as-heat-treated alloy while the irradiation decreased the DBTT. The irradiation effect on the DBTT in the model ferritic alloys containing the different levels of P is discussed in light of embrittling or toughening effects caused by the changes in the P or C segregation, and hardness.