Abstract To study phase stability and effects of alloying elements during neutron irradiation, especially the additional effect of aluminum, model alloys were neutron-irradiated in FFTF/MOTA. Fe-10Cr-3Mn, Fe-5Cr-5Mn, Fe-3Cr-7Mn, Fe-10Cr-15Mn and Fe-10Cr-15Mn-3Al alloys were annealed at 1000°C for 30 min. Compositions of three low-Mn alloys are located close to the alpha ferrite/gamma austenite phase boundary in a ternary phase diagram. Neutron irradiation was performed in FFTF/MOTA cycle 10 to 20 dpa at 600°C. After irradiation it was found that all of the alloys were composed of ferrite/austenite dual phases or single ferrite phase. Voids were formed only in the austenitic phase, and most of the voids were coated with ferrite shells. The Fe-3Cr-7Mn alloy showed only dual phase with fine grains of 1 μm or less in size. On the other hand, Fe-10Cr-15Mn showed a larger amount of austenitic phase and large numbers of voids were formed in the austenitic phase, in which the swelling was about 2.3%. By the addition of Al, the extent of ferritic phase increased. These results indicate that alloying compositions corresponded to dual phase region the extent of which depends on the amount of Cr and Mn even before irradiation and that the formation of ferritic shell phase around voids come from the preferential Mn depletion of void sinks. Furthermore, Al additions are effective for ferritic phase stabilization.