Mesocrystals and single-crystals of MnCO3 have been successfully synthesized through the same reactants with different concentrations by hydrothermal method.The method allows a controlled formation of the self-assembled structure with different crystal types by varying the concentration of reactant (C6H12O6) in a mixed solution containing KMnO4 and at temperature 120℃.Types of as-prepared crystals has been studied and correlated.Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigation indicated that the as-prepared mesocrystals were of different shapes and exhibited tendency for transformation to rhombohedral single-crystals while decreasing the concentration of C6H12O6.By carefully analyzing the mesocrystals of difformity and the intermediates during the crystal growth process,we found that the growth of MnCO3 single-crystals under the given growth conditions follows a nonclassical crystal growth process in which a mesoscale self-assembly of nanocrystallites is included during crystal growth.Such a mesoscale self-assembly process could be a common phenomenon for growth of some single-crystals with extremely low solubility.