The reactor pressure vessel (RPV) is the key component in the nuclear power plant, which is considered irreplaceable and can be the life-limiting feature of the operation of nuclear power plant if its mechanical properties degrade sufficiently. High temperature gas-cooled reactor (HTGR) has perfect inherent safety, which is intended to be one of the fourth generation advanced nuclear reactors. However, HTGR has different service temperature with pressurized water reactor (PWR), that the service temperature of HTGR is 250℃ and that of PWR is 290℃. So the irradiation behaviour of RPV in HTGR is expected to be investigated. In this wok, 3 MeV Fe-ion irradiation was performed on Chinese A508-3 reactor pressure vessel steel which is employed by high-temperature gas-cooled reactors and pure Fe under room temperature (about 25℃) and high temperature (250℃). The ion doses were 0.1, 0.5 and 1.0 dpa for both room temperature irradiation and high temperature irradiation. SRIM modeling was performed before irradiation experiments to guide the experimental details. Positron annihilation Doppler broadening (PADB) spectroscopy experiments and nano-indentation tests (to study embrittlement behavior) were conducted for characterization. It is found that after both room temperature irradiation and high temperature irradiation, the densities of defects in the reactor pressure vessel steel and pure Fe increase, and the type of defects could be vacancy-type and solute cluster type from PADB results. The vacancy-type defect density under high temperature irradiation is lower than that under room temperature irradiation. That is because high temperature can recover the defects formed during irradiation. The hardness test results show that for both the reactor pressure vessel steel and pure Fe, the irradiation hardening increases with increasing dose. Compared to room temperature irradiation, high temperature irradiation can produce more solute clusters and fewer vacancy-type defects in the reactor pressure vessel steel. So the irradiation hardening of the reactor pressure vessel steel might be caused mainly by the formation of solute clusters.