Hydroxyapatite (HA) is a bioactive ceramic, employed mainly in bone tissue engineering since it exhibits superior biocompatibility and osteoconductivity. Attempts have been made to synthesize HA nanoparticles with chemical composition, morphology, crystallinity and Ca/P ratio similar to that of natural bone. While wet chemical methods are becoming more popular for synthesis of HA nanoparticles, ultrasound irradiation has shown to be an effective method to increase the rate of production and also to decrease particle size. However, process variables must be carefully selected. In the present study, HA nanoparticles with desirable characteristics have been synthesized by the aid of ultrasound irradiation and characterized by powder X-ray diffraction (XRD) and electron microscopy techniques.