Polyethylene (PE) substrates as one example of polymers were irradiated at 1015 ions/cm2 by the simultaneous use of oxygen (O 2) cluster and monomer ion beams, and their apatite-forming ability was examined using a metastable calcium phosphate solution that had 1.5 times the ion concentrations of a normal simulated body fluid (1.5SBF). The acceleration voltage for the ion beams was changed from 3 to 9 kV. Some irradiated substrates were soaked in 1M-CaCl2 solution for 1 day. Thus obtained substrates were soaked in 1.5SBF at 36.5°C for 7 days. The hydrophilic functional groups such as COOH groups were formed at the PE surfaces by the irradiation. The amount of the functional groups increased with increasing acceleration voltage, but the hydrophilicity of the substrates decreased slightly at 9 kV. This might be attributed to the carbonization of the PE substrate caused by the irradiation at high acceleration voltage of 9 kV. The irradiated PE substrates formed apatite in 1.5SBF, whereas unirradiated ones did not form it. This indicates that the functional groups such as COOH groups induced apatite nucleation in 1.5SBF. The apatite-forming ability increased with increasing acceleration voltage up to 6 kV, but slightly decreased at 9 kV. The apatite formation was promoted by the subsequent CaCl2 solution treatment. When the substrate is soaked in 1.5SBF, the calcium ions retained at the PE surface by the CaCl2 solution treatment are released into 1.5SBF to increase the ionic activity product of the surrounding fluid with respect to apatite.