Hydroxyapatite $(HAp,;Ca_{10}(PO_4)_6(OH)_2)$/alumina composites were fabricated with the addition of fluorides, such as $MgF_2$ and $CaF_2$. In this study, the effect of fluorides on the inhibition of phase decomposition and the mechanical properties of HAp was investigated. Due to the higher solubility of F ion in HAp structure, $MgF_2$ additive was more effective compared to $CaF_2$ additive in the lowering decomposition temperature of HAp. Therefore, the dissociation tendency of HAp was fully inhibited below $1400^{circ}C$. The mechanical properties of HAp composites with $MgF_2$ additive showed higher value (flexural strength: $~170MPa$, Vickers hardness: $~7;GPa$, fracture toughness: $~1.5;MPam^{1/2}$) compared to $MgF_2-free$ composites. The linear thermal expansion coefficient of HAp composites with $MgF_2$ showed the value of $16.4 imes10^{-6}/^{circ}C;at;20~400^{circ}C $.