The retention characteristics of oxygen were investigated mainly using Rutherford backscattering spectroscopy (RBS) after implantation of 5 keV O 2 + ions on boron carbide. The areal density of saturated retention was dependent on the boron concentration. Boron caused oxygen retention to increase. On the other hand, all the boron containing samples, which included boron over 20 at%, had the same oxygen release characteristics: The implanted oxygen began to release above 600°C and almost all the oxygen was desorbed by ∼ 1000°C. For the boronized film which was made with the same method as that in JT-60U, the saturated retention was ∼ 1 × 10 17 O/cm 2 and oxygen was released under the same temperature characteristics.