Abstract: Rhombohedra microcrystallites of magnesium carbonate (MgCO3) have been hydrothermally synthesized by urea and magnesium sources (Mg, MgCl2, Mg(OH)2, MgSO4, and MgO) at 160°C. The experimental parameters such as reaction temperature, time, and reactants were researched to investigate the reaction mechanism. The product synthesized by Mg powder was about 10μm, and the diameters of the products synthesized by Mg(OH)2 and MgSO4 were about 1–2μm. The product could transfer from the mixture of MgCO3, Mg(OH)2, Mg5(CO3)4(OH)2·4H2O to pure MgCO3 by increasing reaction time or raising reaction temperature, which indicates that Mg5(CO3)4(OH)2·4H2O was just the intermediate product, and MgCO3 was the final product. [Copyright &y& Elsevier]