In the present work, irradiation hardening of a low-copper reactor pressure vessel steel (Chinese A508-3) is studied. Specimens of the RPV steel were irradiated with high-energy Fe ions at a terminal of a cyclotron at a low temperature around 173 K to two damage levels of 0.15 and 0.21 dpa. Micro Vickers hardness was measured from the specimens after stepwise thermal annealing at temperatures of 300, 573, 623 and 673 K, respectively. The specimens irradiated to both two doses exhibit obvious hardening. With the increase of the annealing temperature, the irradiation hardening decreases monotonically. Fitting of the Arrhenius plots gives an apparent activation energy of 0.10±0.01 eV for the temperature regime from 573 to 673 K. Positron annihilation lifetime spectrometry (PALS) tests revealed that the average positron annihilation lifetime initially decreases significantly with the increase of annealing temperature due to defect recombination, then increases at temperatures above 573 K due to vacancy agglomeration. Dislocation loops in a high density were observed in the irradiated specimens after annealed at 673 K, and are regarded to be the main reason for the irradiated hardening. Assuming the evolution of the dislocation loops is controlled by the migration and coalescence process of self-interstitial-atom clusters (SIA-clusters) produced in cascade damage, an activation energy E^m (0.55±0.05 eV) was deduced for the migration of SIA clusters in the irradiated steel specimens.