MgO is a typical wide bandgap insulator with its excellent thermodynamic stability, low dielectric constant, and low refractive index for growing various thin film materials. For this application, it is worth developing and understanding suitable growth method for MgO nanostructure. In this study, cubic MgCO3 have been successfully prepared via hydrolysis control of magnesium salt and alkaline solution with common ions to form nanoflake assembly. An optimum hydrothermal process and its shape evolution of cubic MgCO3 and MgO granules are analyzed. The physicochemical properties of the obtained samples are characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier Transform InfraRed (FT-IR) to show high purity and structural uniformity of magnesium compounds.