Two kinds of hot-pressed B4C specimens were irradiated with 15MeV He+ from 4.3×1014 to 1.13×1016ion/cm2 at 400–760 °C, and with 100keV He+ from 1.0×1013 to 2.0×1017ion/cm2 at room temperature. After ion irradiation, the microstructure of specimens was analyzed by transmission electron microscopy (TEM). The ion-irradiation by 15MeV helium ions produced a radiation-enhanced precipitation with Fe-rich composition and the radiation-induced amorphization in the B4C matrix, but the irradiation by 100keV helium did not make amorphization. This result suggests that the inelastic scattering (electron excitation), rather than elastic scattering, plays an important role in amorphization of B4C. The annealing of ion irradiated B4C above 1000°C indicated incineration of precipitated crystals and a formation of a small numbers of helium bubbles.