The morphology- and size-controlled synthesis of MnCO3 nanocrystals was successfully achieved by cationic surfactant-CTAB-microemulsion-mediated solvothermal method. Various comparison experiments with different reactant concentrations and molar ratios between water and CTAB, showed the evolvement law of the morphology and size of the as-synthesized MnCO3 nanocrystals. With slowly increasing the concentration of reactants and/or molar ratio between water and CTAB, the morphology of MnCO3 nanocrystals changed gradually from cube to parallelepiped, and then rhombohedron, whereas the size decreased a little. The effect of the experimental parameters on the shapes and sizes of samples, such as the source of carbonate salts, reaction time, and temperature, were also discussed. X-ray powder diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and high-resolution transmission electron microscopy (HRTEM) were used to characterize the products.