Nano-particle hydroxyapatite (HAp) was directly prepared by a wet chemical precipitation method with the aid of ultrasonic irradiation in solution using Ca(NO3)2, NH4H2PO4 and NH2CONH2 as source materials. The nano-HAp formation rate at different preparation temperatures and under different ultrasonic irradiation powers was measured and the influence of ultrasonic irradiation power on the synthesis kinetics of the nano-hydroxyapatite was investigated. It was found that the nano-HAp content increased with the increase of ultrasonic irradiation power, preparation temperature and reaction time. Under different ultrasonic irradiation power, an Arrhenius relationship was found between the nano-HAp formation rate and preparation temperature. It showed that with the increase of ultrasonic irradiation power from 100W to 200W and 300W, the synthesis activation energy of nano-HAp crystallites decrease from 63.2 KJ/mol to 59.9 KJ/mol and 48.1 kJ/mol respectively by calculation.