The effect of nickel addition on irradiation-induced hardening has been investigated for reduced-activation martensitic steel (RAMS). Specimens were irradiated in the JMTR at 80°C and 220°C up to 0.15 dpa. There was no significant difference in the tensile properties between the steels with and without Ni addition after the irradiation at 220°C, while in the case of the irradiation below 170°C, the Ni-added RAMS showed a more than four times larger irradiation hardening than the steel without Ni addition. The recovery behavior of the irradiation hardening of Ni-added steel on post-irradiation annealing showed two-step recovery behavior; the first step was around 200°C and the second one was around 350°C, while only the single step of recovery was observed around 350°C in the steel without Ni addition. The mechanism of the recovery process of the tremendous irradiation hardening in Ni-added steel is discussed along with the behavior of vacancies, carbon atoms and their complexes investigated by means of positron annihilation lifetime spectrometry.