Impurity segregation to grain boundaries under irradiation is one of the important radiation effects on materials. For this reason, phosphorus segregation to prior austenite grain boundaries in a P-doped 2.25Cr1Mo steel subjected to neutron irradiation and aging is examined using field emission gun scanning transmission electron microscopy (FEGSTEM) with energy dispersive X-ray microanalysis (EDX). The steel samples are irradiated and aged around 270 °C and 400 °C, respectively. The irradiation dose rate and dose are $#x223C;1.05×10-8 dpa/s and $#x223C;0.042 dpa respectively for 270 °C irradiation, and $#x223C;1.75×10-8 dpa/s and $#x223C;0.13 dpa respectively for 400 °C irradiation. The FEGSTEM results indicate that there is no apparent phosphorus segregation during 270 °C irradiation but there is some during 400 °C irradiation.