Hydroxyapatite (HAp) is an important bioactive ceramic that possessing beneficial biocompatibility and osteoconductivity resulting in bonding to human bone tissues. The dry mechanochemical process is widely used to prepare nanometer HAp. However, little research has been carried out concerning the correlation between adding alumina and the structural changes during the mechanochemical process. In this research, special attention was paid to the effect of alumina additive on the crystallite size and lattice strain of nanocrystalline HAp. Characterization was accomplished by using powder X-ray diffraction (XRD). The obtained data demonstrated that the diffraction lines corresponding to the HAp phase became broad and weak with increasing alumina additive. In fact, the nanocrystalline HAp with high crystallinity degree can be synthesized in the proper amounts of alumina additive via mechanochemical method. Furthermore, an increase of alumina additive led to increase in lattice strain and decrease in size of the powder grain.