Abstract The hardness and plasticity of Zr 55 Cu 30 Al 10 Ni 5 metallic glass (MG) is investigated under He+ ion irradiation. After ion irradiation, the softening of Zr 55 Cu 30 Al 10 Ni 5 MG is observed, which is attributed to the spatial homogeneity of free volume and the destruction of the structural backbone induced by ion irradiation. As increase of ion influence, the formation of helium bubbles and the reduction of free volume may be result in the hardening of Zr 55 Cu 30 Al 10 Ni 5 MG. Moreover, the plasticity of Zr 55 Cu 30 Al 10 Ni 5 MG is modified after ion irradiation, which is attributed to the formation of multiple shear bands, propagation of shear bands and spatial distribution of free volume at different ion influence. Highlights • After He ion irradiation, no crystallization is observed, the interatomic distance is reduced and the helium bubbles are formed in Zr 55 Cu 30 Al 10 Ni 5 MG. • Ion irradiation induced the soften of Zr 55 Cu 30 Al 10 Ni 5 MG, and the mechanism of soften was investigated. • The plasticity of Zr 55 Cu 30 Al 10 Ni 5 MG is modified after ion irradiation, which is attributed to the formation of multiple shear bands, propagation of shear bands and spatial distribution of free volume at different ion influence. [ABSTRACT FROM AUTHOR]