Composites of 25 wt.% monoclinic zirconia and 75 wt.% hydroxyapatite (HA) (with and without 5 wt.% ZrF 4) were hot pressed to investigate their mechanical properties and phase stability. HA decomposed to tricalcium phosphate when there was no ZrF 4 present. Addition of ZrF 4 into the composite improved its thermal stability and mechanical properties. Substitution of OH − by F − ions was verified by the change in HA’s hexagonal lattice parameters. A fracture toughness of 2.1 MPa √m and microhardness of 6.9 GPa were calculated for the composite containing ZrF 4.