Abstract: Hydroxyapatite (HA) coatings on carbon/carbon composites deposited by a plasma spraying technique were immersed in a simulated body fluid (SBF). By analyzing the change of calcium ion concentration and pH value, microstructure, phase composition and calcium to phasphorus ratio of the precipitates on HA coatings, the bioactivity in vitro of HA coatings was discussed. The results show that the calcium ion concentration decreased and the pH value increased with increasing the immersing time. The CaO on the coating surface dissolved completely and the α-TCP and β-TCP dissolved slightly. The precipitation deposited on the surface of coatings was mainly composed of crystalline HA, and the coatings had excellent bioactivity in vitro.