In order to evaluate the degradation and lifetime of high-intensity accelerator targets and related equipment, as well as nuclear materials and equipment, it is important to evaluate the effects of radiation damage, transmutation products, spallation products, etc. on mechanical properties and microstructural changes. In this study the effects of helium concentration and displacement damage on microstructural evolution at low dpa and low helium concentration were mainly investigated in austenitic stainless steels 316FR or SUS304 and a high chromium martensitic steel (HCM12A). The 316FR and HCM12A specimens were implanted uniformly with helium at 823 K up to 30 appm-He or 50 appm-He by 50 MeV cyclotron accelerator using energy degraders. The microstructures after the helium implantation were examined by a transmission electron microscopy (TEM) and positron annihilation lifetime measurements. Irradiation hardening behaviors were analyzed using SUS304 and HCM12A steels implanted with He ion up to 100 appm at 823 K with different He/dpa ratios in the HIT ion irradiation experiments and the hardening behavior was examined by nano indentation method. In the irradiation and annealing specimens, these mechanical properties and microstructures were examined to understand the effects of helium production, displacement damage and annealing on microstructural development, and kinetic Monte Carlo (kMC) simulations were also performed to understand the microstructural development, and the results were compared with the TEM observation and positron annihilation lifetime measurements. Important some differences in the microstructural developments such as cavity formation and growth between austenitic stainless steel and martensitic steel were observed in low dpa and low helium concentration conditions. The kMC calculation result of the larger HeV cluster formation was close to the results of bubble formation observed by TEM and the mean lifetime of positron annihilation measurements. Many of HeV clusters and V clusters with large sizes exist, and the hardness of fcc Fe such as 316FR or 304 steel can be expected to be increased, compared to the bcc Fe such as HCM12A steel. The irradiation hardening behaviors up to about 1x10-2 dpa of austenitic stainless steel and ferritic/martensitic steel performed in the HIT experiments irradiated at 773 K are also obtained similar results. The irradiation hardening mechanism and creep behavior due to helium atoms and dpa were also discussed.