The segregation behavior of Fe-Cr-Mn alloys during electron-irradiation was studied using a high-voltage electron microscope at a temperature range between 723 and 823 K. The ferritic Fe-7Cr-3Mn, Fe-5Cr-5Mn, and Fe-3Cr-7Mn alloys and the austenitic Fe-10Cr-15Mn and Fe-10Cr-15Mn-3Al alloys were used. Irradiation-induced segregation was observed for all of the Fe-Cr-Mn alloys. That is, manganese tended to enrich and chromium deplete near grain boundaries. However, both manganese and chromium were enriched to high concentrations not only at grain boundaries but also in the grain so that new phases were formed. Furthermore, the amount of segregation depended on the initial manganese and chromium solute concentration along with the aluminum addition. From these results it is suggested that compositional changes due to enrichment or depletion or both can result in a phase transformation during irradiation, even local phenomena.