Ion irradiations with 6.4 MeV Fe3+ were performed on pure-Fe, Fe-1 at.% Cr, Fe-1 at.% Mn, and Fe-1 at.% Ni to a nominal damage of 1 dpa, at a damage rate of 1 × 10-4 dpa/s, at irradiation temperatures of 473, 563, and 673 K. After irradiations at 473 and 563 K, Fe-1Mn and Fe-1Ni showed significant irradiation hardening, which was due to irradiation induced dislocation loops in high density. In pure-Fe, the dislocation loops were localized in the vicinity of dislocations, while those in Fe alloys were distributed rather homogeneously. This can be interpreted in terms of the interaction between alloying element and dislocation strain field. Irradiation at 673 K resulted in the formation of voids in pure-Fe, Fe-1Cr, and Fe-1Mn. We found that chromium suppressed void swelling.