Amorphous calcium carbonate (ACC) is one of the important precursors of calcium carbonate crystal phases and its transformation plays key roles in biomineralization. In this paper, the transformation of ACC is studied in the presence of magnesium ion, which often appears in the crystallization of calcium carbonate. We use powder X-ray diffraction, Fourier transformed infrared (FTIR) spectroscopy and scanning electron microscopy to monitor this transformation process and find that in high Mg2+ environment, there appears a transformation process with multiple-step self-assembly from ACC into aragonite. To the best of our knowledge, it is the first time the growth of aragonite via the nano-crystal particles assembly and transformation has been discovered, which may have a great significance in understanding the mystery of aragonite formation in an ocean. (C) 2012 Elsevier B.V. All rights reserved.