Micron-grade calcium carbonate (CaCO3) crystals are by-products in the preparation of (NH4)(2)SO4 fertilizer using CaSO4 center dot 2H(2)O, NH3, and CO2. Since ultrasound can make crystals smaller and their morphology quite complex, the morphological change rules need to be studied to make meaningful use of them. In the present work, the morphologies of CaCO3 crystals precipitated from the ultrasound CaSO4-NH3-CO2-H2O system were analyzed in different conditions. It was found that ultrasound can make the crystals smaller in nucleation or the early growth stage, or it can make the shapes diversified and dispersed by influencing the solution property and in the aging process. In an ultrasound environment, crystals may be square, spheroidal, ellipsoidal, pisciform (spindle), hexagonal, carambola-like, or scattered particles. High-concentrations of NH3 and CO2 facilitate CaCO3 crystals becoming smooth balls, while high-intensity ultrasound and a high-concentration of foreign Na+ ions help to turn CaCO3 into tiny square particles or a clustered aggregate of tiny particles with rough surfaces.