Hydroxyapatite, HA, with crystal size smaller than 10 nm was synthesized by dropwise addition of calcium nitrate and ammonium phosphate under controlled conditions. Powder was composed by agglomerates of nano-particles with specific surface area higher than 200 m2/g. HA samples had a main pore size distribution centered at 13-15 nm with a secondary mode centered at 3-5 nm. High resolution TEM revealed that individual particles had nearly spherical shapes and were mainly composed of a crystalline core and an amorphous external component. EXAFS measurements confirmed that the short range order of those HA nanocrystals was similar to that of HA sintered at high temperature. HA powder with small particle size incorporates in its structure more water and carbonate ions than HA prepared in normal conditions.