Previous studies have reported preparation of nanometer hydroxyapatite (HA) by sol-gel or precipitation methods, where the products were washed by aqueous or non-aqueous liquids that may modify the properties of the nano particles.  Objective: This study evaluated properties of nano HA prepared by a spray drying method where the HA product was not exposed to any liquid after its formation.  Methods: The spray drying apparatus comprised of a nozzle that sprayed a calcium phosphate solution in the form of a fine mist into a stream of heated air flowing though a glass column.  The water in the mist was evaporated by the time the mist reached the end of the column and an electrostatic precipitator collected the fine particles.  The nano HA products were characterized by powder X-ray diffraction (XRD), high resolution transmission electron microscopy (TEM), Fourier transfer infrared (FTIR) spectroscopy, and solubility measurements.  Results: XRD patterns suggested the material was amorphous, exhibiting a single broad peak at 30.5º 2θ.  However, TEM analysis showed that the particles, some of which were 5 nm in size, exhibited well ordered HA lattice fringes and small area diffraction patterns were indicative of HA.  FTIR showed patterns of typical of HA with small amounts of HPO₄^2-. The thermodynamic solubility product of the nano HA (given as pKsp) was 103 ± 4 in the pH range of 5 to 6 which is significantly more soluble than the value of 118 for crystalline HA. Conclusion:  HA particles as small as 5 nm in size, prepared by a spray drying method, showed amorphous XRD patterns but exhibited well-defined lattice fringes in TEM.  The nano HA was significantly more soluble than conventional crystalline HA. Supported by NIDCR grant DE11789, ADAF, and NIST.