Ferritic/martensitic (FM) steels, F82H and Optimax-A, were irradiated up to 11.3 dpa/1175 appm He in Target-3 of the Swiss Spallation Neutron Source (SINQ). To investigate helium-induced hardening effect, micro-hardness and microstructure of the irradiated specimens were investigated in as-irradiated condition and after annealing at 600 °C. The results show that the hardening in the as-irradiated specimens increased linearly with √dpa at low doses and more rapidly at higher doses, which is contributed by both defect clusters and helium bubbles. After annealing, although the hardening decreased greatly, a significant amount remained, which is proportional to √CHe (CHe is helium concentration) and essentially contributed by helium bubbles. From the results of the annealed specimens, it is deduced that the helium bubbles with sizes 1-1.5 nm are weak obstacles with barrier strength about 0.1. The significant hardening in the annealed specimens can be attributed to the high density of bubbles. © 2010 Elsevier B.V. All rights reserved.