Beryllium is planned to be used as a neutron multiplier in the Helium-cooled ceramic breeder(HCCB)test blanket module(TBM)which is the primary option of the Chinese TBM program.The character of the microstructural change of beryllium under irradiation is a determining factor in understanding of the processes resulting in the degradation of physical-mechanical properties of the material.Beryllium metal was irradiated with 100 keV Ar+ions to a dose of 1×1017/cm2 at room temperature.The microstructural change of Beryllium has been investigated using transmission electron microscopy(TEM).After the irradiation,a high density of black-dot-like defects were observed in the sample,some of which were resolved as small dislocation loops.Gas-filled bubbles with an average size of about 33.7 nm were also observed in the damaged zone.Ion irradiation induced swelling at room temperature was also discussed.