The effect of alkaline hydrothermal temperature on the transformation of induction heating deposited monetite coatings to hydroxyapatite coatings on the H2O2 treated C/C composite was investigated. These monetite coatings were hydrothermally treated for 4h at 353K, 373K and 393K in a 50ml autoclave with 0.1M alkaline solution. After hydrothermal post-treatment, the structure, morphology and the chemical composition of these HA coatings were characterized with XRD, FTIR, SEM and EDS. A scratch test was conducted to measure the strength of the adhesion of the coatings to the HT-C/C substrate. The results showed that only part of hydroxyapatite phases at 393K could be transformed from the monetite coatings on HT-C/C substrate, while pure hydroxyapatite phases were observed to form in the as-converted coatings with critical loads of 45.9N and 51.1N respectively when the alkaline hydrothermal post-treatment treating temperatures were 353K and 373K. The reasons for these results were suggested.