The microstructures of a simple ternary Fe-16Cr-15Mn alloy and a solute modified Fe-12Cr-18Mn-0.26C WVN alloy were examined after irradiation with 4-MeV Ni ions (single-beam irradiation) and with 4-MeV Ni ions with simultaneous injection of energy-degraded 1-MeV He ions (dual-beam irradiation). The microstructure prior to irradiation revealed that the ferrite phase was dominant in the Fe-16Cr-15Mn alloy in spite of the prediction of the Schaef-fler diagram, while the Fe-12Cr-18Mn-0.26C WVN alloy contained only austenite. In the irradiated Fe-16Cr-15Mn alloy, formation of a lenticular-shaped phase was observed following irradiation to 15 dpa or more. Electron diffraction analysis demonstrated that this was an austenite phase. Austenite formation was observed in both dual-beam and single-beam irradiations. The fraction of this phase increased abruptly in the early stages of irradiation. The volume fraction saturated to about 70% following irradiation beyond 20 dpa, but the saturation appeared to occur faster in dual-beam irradiations than in single-beam irradiations. Cavities were formed uniformly only within the austenite phase. Local swelling reached as high as 50% following irradiation at 773 K to 120 dpa. In the Fe-12Cr-18Mn-0.26C WVN alloy, the austenite structure was stable during irradiation, and no radiation-induced phase transformation was observed. Cavity formation was strongly suppressed in this alloy. Small cavities with a number density less than 1022 m-3 were observed following irradiation at 773 K to 35 dpa. The calculated swelling from these cavities was estimated as high as 0.4%, which was much lower than the swelling level in dual-beam irradiated 316 stainless steel.