To understand mechanistically the influence of cyclic temperature change on microstructural evolution in Fe-Cr-Ni austenitic alloys, temperature-cycle neutron irradiations of 473 K/673 K and 573 K/723 K were carried out at JMTR (0.9-1.4 x 10(24) n/m(2) (> 1.0 MeV)). Suppression of interstitial loop formation was remarkable. Only a few large voids were formed in Fe-Cr-Ni and Fe-Cr-Ni-P alloys, but unexpectedly large void swelling (1.5-1.6%, 9-11%/dpa) was observed in Fe-Cr-Ni-Ti alloy. Rate theory of defect clustering for temperature-variant irradiation indicates that vacancy-rich condition appears temporarily at the beginning of the high-temperature period due to the reclustering of the vacancies formed in the low-temperature period through the reactions with radiation-induced vacancies and interstitials.