In this work, a solution-annealed specimen of a low-activation Fe-Cr-Mn alloy was irradiated with 92 MeV Ar ions at 450degC to a dose of 1.7times10²¹m^-2 which was expected to produce a peak displacement damage of 90 dpa. After irradiation, damage microstructure was investigated from the cross-sectional specimens using a transmission electron microscope. High number density cavities were observed in the peak dose region. Size of cavities was the largest at the peak displacement damage. Formation of alpha-phase was found at a grain boundary in the peak dose region. Well-dispersed carbide particles was found in the matrix. The carbide/matrix interfaces supplied favorable site for growth of large cavities, whereas only small cavities were found inside the particles.