Nanosized hydroxyapatite (HA) powders have been synthesized by an inverse microemulsion system using cyclohexane, an anionic surfactant and aqueous solutions of calcium nitrate tetrahydrate and biammonium hydrogen phosphate in the presence of anionic and caionic polyelectrolyte with the inducement of ultrasound irradiation. Prepared HA in presence of anionic polyelectrolyte has a different morphology from samples which are synthesized in presence of cationic polyelectrolyte. Cationic polyelectrolyte (poly diallyl dimethyl ammonium chloride PDADMAC) leads to formation of needle-like HA (10nm in diameter and 100nm in length). Formation of HA at room temperature was confirmed by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). Size and morphology of the HA samples were characterized using transmission electron microscopy (TEM).