Ferritic /martensitic (FM) steels are among the most important materials in various nuclear applications. As radiation-induced segregation (RIS) is one of the major factors for embrittlement of nuclear materials, thereofre it is essential to study the RIS in FM steels. A comprehensive study has been performed on F82H, Eurofer 97, and ODS Eurofer steels after irradiation at SINQ by combining APT and TEM techniques. The study reveals that after irradiation, the RIS at low-angle GBs in F82H is less pronounced as compared to the random high-angle GBs, while in ODS Eurofer the RIS is similar. It was also found that the element segregation extent is not influenced by the GB CSL structures. The results confirmed that, as reported in literature, the small interstitial elements, such as C, N, P, exhibit GB segregation behaviour. Whereas, the GB segregation of substitutional elements depends on their atomic size. The undersized elements are enriched, and the oversized elements V and W are generally depleted at GBs. In some special cases, oversized elements are also enriched at GBs after irradiation, which may be caused by the “solute drag effect” of O or N. For the elements with giant sizes (with a volume over 2 times of Fe) such as Ca, Sc and K, their RIS behaviour was not known in literature, and in this work they showed strong enrichment at GBs. Further, this study also revealed that the segregation of these elements at interfaces between phases of chromium carbides, vanadium nitrides, and the matrix is essentially similar to that at GBs, except for Ti. The radiation-induced clustering of solutes was also studied. Two kinds of clusters were investigated in this study, including oxide clusters in ODS Eurofer and nano-clusters (N-clusters) with Si, Mn, Ti, Sc, Ca and K. In F82H, with increasing irradiation temperature, the N-clusters are changed from dilute, nonuniform-size and -shape to more uniform-size, compacted, and sphericial shape. Comparing with that of F82H, N-clusters have a lower number density and bigger size in Eurofer 97 irradiated under the same condition ~20 dpa/ ~300 °C. In ODS Eurofer, N-clusters were only observed in specimen irradiated at 300 °C. Some oxide clusters in the specimen irradiated at 480 °C contain a high portion of Si, Mn, Ti, Sc, Ca, and K. The number density of oxide clusters increases and their size decreases with irradiation temperature. Last but not least, the concentrations of spallation transmutants were quantified in detail. The results agree well with that of the neutronic calculations. This comprehensive study of irradiation effects on GB segregation and solute clustering revealed a number of novel features in FM steels. The investigation on the transmutation elements not only greatly contributes to spallation materials technology, but also broadens the basic understanding of atomic size effect on GB segregation. The comparison between the RIS behaviors in F82H / Eurofer 97 and ODS Eurofer demonstrated that the oxide clusters or oxygen could promote the segregation of some elements such as V, W and Ti, which is of significance for the application of ODS steels in intensive irradiation environments.