Dry mechanosynthesis is an efficient technique to synthesise nanocrystalline calcium deficient hydroxyapatites (CDHA). The mechanisms underlying a mechanochemical reaction are different from those triggering a dissolution mediated process, and this can have an effect on the structural features of the product. In this work, a nanocrystalline CDHA with Ca/P molar ratio of 1.5 obtained by means of dry mechanosynthesis of calcium oxide and dicalcium phosphate dihydrate was analysed. Spectroscopic techniques confirmed the presence of hydrogen phosphate (HPO4 2−) groups and a non-apatitic environment of the phosphate ions and disordered hydroxyl groups due to the nanometric size of the crystals. Lattice parameters of mechanosynthesised CDHA showed a small increase in the a lattice parameter (9.4418(20)A˚ ) and a small decrease in the c lattice parameter (6.8745(17)A˚ ), in agreement with the values reported in the literature for precipitated CDHAs. A prolonged milling resulted in an increase of the crystallinity of the CDHA and its partial decomposition into -TCP by the loss of OH− and HPO4 2− groups.