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The effects of varying-temperature irradiation on mechanical properties and microstructure of reduced-activation 9Cr martensitic steels (RAMS) were investigated by means of micro-Vickers hardness tests and positron annihilation lifetime spectrometry. In case of stepwise increasing-temperature irradiation of 473/623 K, irradiation hardening accumulated at the lower temperature still existed after the elevation in temperature, while in case of the 493/693 K varying irradiation, the low temperature irradiation hardening disappeared after the elevation in irradiation temperature. In both the varying-temperature irradiations, it was observed that microvoids disappeared after the elevation in irradiation temperature. Result of a computer simulation of the evolution of defect clusters using rate theory were in good agreement with the experimental results when I-clusters were considered to be a factor controlling irradiation hardening.