Nuclear grade 304 stainless steel was irradiated by 3.5 MeV Fe ions,with fluxes of 3.05E+ 15 ions/cm2 and 1.55E+ 16 ions/cm2.Irradiation effects were studied by positron annihilation spectroscopy (PAS),transmission electron microscope (TEM)and nanoindentation techniques.PAS results showed that different types of defects were produced after irradiation and that there was significant variance in defects formed when the samples were subjected to different irradiation doses.TEM char-acterization showed that the irradiation-induced dislocation loops enlarged in average size,but decreased in number density at higher irradiation doses.Nanoindentation test showed obvious irradiation hardening phenomenon,which was in good agreement with the PAS and TEM results.Irradiation hardening effect increased with an increase in irradiation dose and saturation occurred with an increase in irradiation dose from 3.2 to 16 dpa.Further statistical analysis showed that barrier strength of the Frank loop depends on the loop size and density produced by the ion irradiation.