Properties of lithium orthosilicate, Li 4SiO 4, as a tritium breeding material will be substantially affected by structural change due to irradiation. An infrared absorption at 1040 cm −1 appeared in the FT-IR PAS spectra of Li 4SiO 4 irradiated with high energy oxygen ions. The semi-empirical molecular orbital method, MNDO, was applied to the interpretation of the changes of FT-IR PAS spectra of irradiated Li 4SiO 4. Vibrational frequencies and force constants have been calculated for the silicate clusters model: SiO 4 isolated tetrahedron, Si 2O 7 dimers, Si 2O 6 chain links, Si 2O 5 sheet units, and Si 2O 4 three-dimensional (3D) framework units. The calculations show that the Si O Si stretching vibrations are dependent on the degree of polymerization of the tetrahedron and connection of Li atoms. By comparing the FT-IR PAS spectra obtained experimentally in this study with the calculated spectra, the absorption at 1040 cm −1 due to the irradiation was assigned to the Si O Si stretching vibrations in the Si 2O 5 sheet and Si 2O 4 3D-framework structures.