Abstract: The principle and method of nanometer calcium carbonate preparation are introduced. Under the ultrasonic condition, the effects of initial carbonizing temperature, density of Ca(OH)2 and flow rate of CO2 on nanometer calcium carbonate synthesis are investigated. The results show that the ultrasonic can strengthen the nanometer calcium carbonate preparation, improve the efficiency of mass and heat transfer, greatly enhance the supersaturation of calcium ions in the solution and induce rapid and uniform nucleations of the calcium carbonate. Moreover, the initial carbonizing temperature can be enhanced about 5°C because of ultrasonic, which can shorten the preparation time and improve the synthetic efficiency. On the basis of experimental investigation, the nanometer calcium carbonate powders with diameter of 20~30 nm were synthesized steadily under optimal process conditions in a homemade ultrasonic reactor. The results indicate that high-quality nanometer calcium carbonates with smaller and homogeneous size can be obtained by ultrasonic method.