In order to investigate the effects of helium on the microstructural evolution in Fe–Cr–Mn (W, V) alloy during three irradiation modes of electron beam, electron/He⁺-ion dual-beam and electron irradiation after helium pre-injection, in situ observations were carried out using a high voltage electron microscope connected with an ion accelerator. The interstitial-type dislocation loops and small voids were formed in the early stage of irradiation. The average void size was smaller and void number density was higher under irradiation condition by electrons after helium pre-injection and by dual-beam in comparison with the case of electron irradiation. Irradiation-induced grain boundary segregation of Cr and Mn was suppressed due to existence of helium. From these results it is suggested that helium strongly influences dislocation structural development and void nucleation during irradiation. The effect of interactions between helium and point defects is discussed.