We soaked porous ultrahigh molecular weight polyethylene (UHMWPE) treated with oxygen plasma in simulated body fluid (SBF) with higher pH in comparison with that of conventional SBF, and precipitated calcium phosphate in the pores of the porous UHMWPE by raising temperature of the SBF. By soaking in physiological SBF, apatite formation was induced and the apatite covered the whole surface of the above-mentioned composite. High adhesive strength between the formed apatite and the composite was obtained by conducting the oxygen plasma treatment as a pre-treatment.