Structural changes in thin amorphous SiO2 (a-SiO2) films induced by ion bombardment were examined with FT-IR and ESR techniques. 10MeV H+, 15MeV He+, 2MeV Li+, 4MeV C+ and 30MeV Si6+ ions, which traverse the 200nm thick of SiO2 layer without chemical reaction with SiO2, were used as ion beams with dosage of 1012−1016cm−2. A decrease in the frequency of one peak in the infrared absorption spectrum from 1078cm−1 to 1041cm−1 and an increase of a second peak from 805cm−1 to 830cm−1 were simultaneously observed with cumulative ion dose. The concentration of E' centers has a maximum as a function of dosage in the region of 1–5×1019cm−3. We propose that a reduction of the Si-O-Si bond angle followed by displacement of oxygen is induced with ion bombardment.