The damage structures in Cu and Cu based alloys neutron-irradiated at 200℃ were examined by transmission electron microscopy. Large dislocation loops of complex structure and small dot defects were formed in the specimens. The large dislocation loops were interstitial type. On the other hand, the small dot defects were vacancy type. They were stacking faulted tetrahedra. The damage structure in these metals evolves as follows. Interstitial atoms produced by neutron irradiation aggregate and grow to large complicated loops which will become finally dislocation lines by intersection of them with further irradiation, by absorbing interstitials subsequently produced. By the consumption of the interstitials to the sinks, vacancy concentration increases locally to be high enough to form vacancy defects
