Abstract The effect of ethanol on the crystallization and phase transformation of MgCO3·3H2O in a MgCl2–CO2–NH3·H2O system was investigated at 40 °C. The obtained crystals were characterized by X-ray diffraction and scanning electron microscopy, and the phase transformation behaviors of the MgCO3·3H2O crystals were studied quantitatively using an established thermal analysis method. The results showed that ethanol played a key role in inhibiting the aggregation of MgCO3·3H2O crystals because the adsorption of ethanol molecules on nuclei surfaces was preferential to that of water molecules. Active Mg2+ ions could be separated from the nuclei surfaces when 40% ethanol in solution, leading to the formation of more stable, flower-like crystals of 4MgCO3·Mg(OH)2·4H2O. The results of this study could contribute to the development of methods for controllable crystallization of MgCO3·3H2O.