Irradiation behavior of model austenitic steels is investigated to establish mechanistic modeling of swelling and other property changes under irradiation. Neutron irradiation experiments were performed using Material Open Test Assembly (MOTA) at 693, 793 and 873 K to 35 to 40 dpa in the Fast Flux Test Facility (FFTF) cycles 10 and 11. Cavity formation and dislocation development were quantitatively estimated to evaluate the effect of temperature history on the microstructural evolution under irradiation in fission reactors. Fine dislocation loops observed in the matrix of the alloys irradiated in cycle 10 were considered to come from the low-temperature irradiation during the reactor shut-down at the end of the cycle. Very few small loops were detected in identical specimens irradiated in FFTF cycle 11 where the reactor power and temperature were simultaneously decreased a short time during the shut-down.