The effects of ultrasonic and Baumé degree of Ca(OH)2 suspension on the size of nano-CaCO3 prepared by carbonization were studied. The results indicated that ultrasonic could improve the dispersibility of powder, reduce the aggregation and decrease the particle size of nanometer calcium carbonate. The fine particles synthesized at 40 kHz were smaller but rougher than that obtained at 20 kHz. The frequency hardly influenced the morphology of nanometer calcium carbonate. Moreover, the lower Baumé degree of Ca(OH)2 suspension was adopted, the smaller the size of nano-CaCO3 were obtained. And in the process of carbonation, we were using SEM and XRD to study the nucleation and growth mechanism, it indicated that amorphous calcium carbonate particles acted as nuclei in the initial stage oriented absorption calcium hydroxide to form wirelike substances. In the end, these botryoidal shape crystals were disaggregated into calcium carbonate with the particle diameters of 30nm.