Abstract Simple model alloys of Fe-16 Ni-15 Cr- xC- yTi- zNb were irradiated at 673 K up to 2.5 dpa in the experimental fast breeder reactor JOYO. TEM examinations were made to investigate the separate and synergistic effects of C, Ti and Nb additions on the development of microstructure. Additions of C decreased swelling resulting from decreased cavity number densities. Ti or Nb alone changes little swelling. Ti significantly decreased cavity number densities. C additions to the alloys with Ti and Nb increased the size and number density of cavities; the number density of smaller sized cavities (for example, < 5 nm in diameter) was remarkably increased. Faulted dislocation loops were characteristically formed by additions of Ti and Nb, and became smaller in size and much more circular in shape with fluence. This tendency was enhanced by the further addition of C, where unfaulted loop or dislocation tangling was rarely observed and no precipitation of TiC or NbC occurred.