The surface of synthetic calcium hydroxyapatite Ca-10(PO4)(6)(OH)(2) (CaHAP) particles was treated by repeated modification with hexamethyldisilazane [(CH3)(3)Si](2)NH (HMDS) in hexane and thermal treatment and the surface of the modified CaHAP was characterized by various means, No remarkable change in XRD patterns or in particle shape by the modification was observed. The width of the CaHAP particles gradually increased with repeating the modification. FTIR results indicated that HMDS reacted with surface P-OH groups of CaHAP to yield surface Si-(CH3)(3) groups. The surface of the modified CaHAP was hydrophobic. The surface Si-(CH3)(3) groups turned to three kinds of surface Si-OH groups by treating the modified materials at 500 degrees C in air. These formed surface Si-OH groups and the remaining surface P-OH groups reacted with HMDS by repeating the modification, resulted in the increase of the surface Si atoms. The modified material having surface Si-(CH3)(3) or Si-OH groups adsorbed much less CO2 than the unmodified one,