In this study, nanocrystalline hydroxyapatite (HA) powders were prepared using different natural and synthetic sources. Nanocrystalline HA powders were synthesized successfully via sol- gel method using phosphoric pentoxide (P 2 O 5 ) and calcium nitrate tetrahydrate (Ca(NO 3 ) 2 .4H 2 O) raw materils and also, by sintering of some natural sources including human, bovine, horse and camel bones. X-ray diffraction (XRD) technique was utilized to confirm presence of the desired phases in the structure and also, to determine the crystallite size and crystallinity of the prepared HA powders. The crystallite sizes were calculated by scherrer equation and a new method called modified scherrer and the results were compared. To evaluate the shape and size of the particles and crystallites, a transmission electron microscopy (TEM) was applied. The functional groups of the prepared HA powders were analyzed by Fourier transform infrared (FTIR) spectroscopy. Chemical and elemental analysis of the prepared HA powders were also carried out by X-ray fluorescence (XRF) and energy dispersive X-ray spectroscopy (EDS). Bioactivity of different HA powders was investigated by soaking them in simulated body fluid (SBF) for various periods (14 and 28 days). The concentration of calcium ions in SBF and the content of pH of SBF were determined for various periods by ICP method and pH meter, respectively. Also, surface morphologies of prepared HA samples before and after soaking in SBF were studied using scanning electron microscopy (SEM). In order to verify the biocompatibility of these HA powders, MTT assay was applied. Obtained results showed that in the sintering temperature of 700 ?C, dominant phase in the prepared products is hydroxyapatie and the crystallinity degrees of powders prepared from natural sources were much more than synthetic powder. Determination of crystallite size of powders by scherre, modified scherrer and TEM methods confirmed that powders have the crystallite size of less than 100 µm. Moreover, spectra of FTIR analysis revealed that the most characterized functional groups in prepared HA powders were PO 4 3- , OH - and CO 3 2- , showing all characteristic absorption peaks of pure or stoichiometric HA. Elemental analysis showed that the major element components were phosphorus (P) and calcium (Ca), which in natural samples were with some other trace elements. Also, based on this study the Ca/P molar ratio of sol- gel sample was calculated to be about 1.75, while the Ca/P molar ratios of natural HA samples were considerably higher than 1.67. The SEM images of samples before soaking in SBF exhibited the formation of HA powders containing aggregates rough and granular to dense surfaces, whereas the SEM studies of HA samples after soaking in SBF depicted the tiny agglomerated bone-like apatite particles. Also, Biocompatibility evaluations by MTT method showd that HA particles derived from human and horse bones could stimulate cell proliferation at all concentration ranges and they did not have any cytotoxicity, as compared to other samples. Keywords : synthetic hydroxyapatite, natural hydroxyapatite, sol- gel, sintering, crystallite size, Physico- Chemical characterization, biocompatibility.