Synthesis of hydroxyapatite by hydrothermal route starting from Ca(NO3)2.4H2O and (NH4)2HPO4 was realized. The composition of synthesised sample is placed in the Ca10(PO4)6(OH)2-CaO.SiO2 subsystem. Silicon oxide was used either as amorphous silica or as wollastonite. The samples were hydrothermally heated at the temperature of 140°C with a plateau ranging from 1 and 4 hours. In these conditions it was noticed the stepwise formation of the hydroxyapatite intermediary compound being CaHPO4. The crystalline degree of obtained hydroxyapatite is low, but it can be improved by thermal treatment at temperatures over 700°C. At higher temperatures than 10000C and respectively 1300°C the formation of some anhydrous compounds that are a-2CaO.P2O5, b-3CaO.P2O5 and a-3CaO.P2O5 was observed.