Abstract: To study the effect of dose rate on irradiation hardening of the reactor pressure vessel steel (A508-3), the specimens of RPV steel were irradiated by 3.5 MeV Fe ions to four doses (0.1, 0.3, 1, 3 dpa) at three different dose rates (0.1, 0.5 and 1 dpa/h), respectively. Nano-indentation technique was used to characterize the change in hardness after irradiation. At high dose rate(1.0 dpa/h), the results indicated that irradiation hardening increases rapidly with dose and tends to saturation at the dose higher than 0.3 dpa. Compared to that with high dose rate, the hardness increment showed similar behavior at low dose rate(0.1 dpa/h) and medium dose rate(0.5 dpa/h), but it still increased gradually at dose higher than 0.3 dpa. When dose is lower than 0.3 dpa, a small difference showed in irradiation hardening under different dose rates irradiation, but the difference became significant when the dose was higher than 0.3 dpa. At present dose regime, the irradiation hardening was found to be inversely proportional to the dose rate.