X-ray diffraction was used to characterize the dependence of mean crystallite size and crystallinity on the thickness of coatings. A fall in mean crystallite size but a rise in crystallinity with increased thickness was observed. The reason might be due to the differences in cooling rate of partially molten particles of hydroxyapatite. The thicker the coating, the longer the cooling time. The longer cooling time was beneficial to the occurrence of recrystallization. It was found that the critical thickness of recrystallization (rc) was about 20 microns.