It had been reported that the effects of cyclic temperature change on the radiation induced microstructural evolution in Fe–Cr–Ni alloys were very strong and complicated. In order to understand these results, accumulations of defects under varying temperature irradiations were numerically calculated based on the rate theory for defect clustering. The results indicate the microstructural evolution under varying temperature irradiation as follows. Vacancy-predominant condition appears after changing the temperature from low to high due to the decomposition of small vacancy clusters, which had formed during the low temperature irradiation, by reacting with interstitials produced by the irradiation at the high temperature. Interstitial clusters, therefore, shrink, while vacancy clusters grow by absorbing the excess vacancies. In the case of temperature change from high to low, on the other hand, the interstitial-predominant condition is held throughout the whole irradiation period, and hence interstitial clusters grow, while vacancy clusters shrink. In the cyclic temperature irradiation, the large interstitial and vacancy clusters are difficult to be formed because these clusters grew and shrank repeatly during the temperatures changed periodically.