The main purpose of the present work was to investigate the influence of urea concentration on hydroxyapatite (HAp) synthesis. Calcium nitrate and ammoniumdihydrogenphosphate were used as Ca and P sources. A homogeneous precipitation agent – urea - was applied in the field of ultrasonic irradiation. Different Ca : P : urea molar ratios were applied and the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and infrared spectroscopy (IR). The results reveal the influence of the urea concentration on the phase composition of the system, crystallite size and the particle morphology. Different Ca : P : urea molar ratios led to various system formation: from biphasic OCP and HAp to monophasic HAp. The estimated HAp crystallite size was 24.4 nm. An explanation of the mechanism of the HAp synthesis is given, regarding the formation of the intermediate and final product during ultrasonic irradiation.