There is also a growing interest in developing multifunctional hard bone tissue analogs exhibiting high biocompatibility and osteoconductivity together with therapeutic effect. Selenium (Se), in that respect, is an effective therapeutic agent with promising antioxidant and anti-carcinogenic effect when used in proper doses. Here, selenium-doped HAp (HAp:Se) particles have been synthesized by modified aqueous precipitation method using calcium (Ca(NO3)×4H2O) and phosphate ((NH4)2HPO4) salts with sodium selenite (Na2SeO3). The effect of Se dopant in different amounts and calcination temperature (800-1100°C) on the physical, chemical and crystal structure of resultant HAp have been investigated in detail. Complete chemical investigation was performed with spectroscopical analyses including fourier tranform infrared (FTIR) and x-ray photoelectron spectroscopy (XPS) to elucidate the mechanism and chemical nature of Se doping in HAp. Meanwhile, x-ray diffraction (XRD) studies by rietveld refinement in combination with transmission electron microscopy(TEM) studies have conducted to elucidate the changes in the HAp crystal structure upon Se doping. A correlation between Se incorporation site in HAp crystal and Se dopant amount has been established, which critically regulates the therapeutic activity of Se.