Irradiation-induced hardening and loss of ductility of ferritic/martensitic materials envisaged for future fusion reactor is still difficult to understand. In particular, helium (He), produced by transmutation by the fusion neutrons of 14 MeV, is known to impact mechanical properties, but its effect at the microstructural level is still unclear. Molecular dynamics simulations of the mobility of an edge dislocation in iron (Fe) are performed to study the effect of He, either as a gas in solid solution or in cavities. Obstacle to the dislocation, the cavity, in the form of a void or a He bubble, is compared to a ao [100] dislocation loop, all being 2 nm in size. Results show that He in solid solution up to 1.0 at.% has a little effect on the dislocation mobility. Conversely, the cavities and the ao [10 0] dislocation loop are strong obstacles to the passage of the edge dislocation. Interestingly He bubbles present a lower obstacle strength than voids for low He contents, while for high He content the bubble promotes loop punching, which induces a strong resistance to the passage of the dislocation. (C) 2007 Elsevier B.V. All rights reserved.