Hydroxyapatite (HA) was coated onto titanium substrates using radio frequency sputtering, and the sputtered films were crystallized under hydrothermal conditions at 110-170 degrees C at pH values of 7.0 and 9.5. The crystallite size, the remnant film thickness, and the surface morphology of the films were observed using X-ray diffraction, energy dispersive X-ray spectroscopy, and scanning electron microscopy, respectively. The crystallite size increased with the process temperature, and reached 123.6 nm (pH 9.5 and 170 degrees C) after 24 h. All of the crystallite sizes of the film treated at pH 9.5 were higher than those treated at pH 7.0 at each process temperature. The film treated at pH 9.5 retained more than 90% of the initial film thickness at any process temperature. The ratio of the film treated at pH 7.0 did not reached 90% at less than 150 degrees C, and tended to increase with the process temperature. (C) 2010 Elsevier B.V. All rights reserved.