The mechanism of forming the nanometer Mg,Al-hydrotalcite and its crystallization process based on the crystallization chemistry is studied. The nanometer Mg,Al-hydrotalcite was prepared in precipitation by the one-step reaction method at atmospheric pressure. Moreover, the relationship between the nucleation technology process and the size of Mg,Al-hydrotalcite samples is discussed according to the results of TEM, UV and XRD. Later, based on the theoretical model of anionic coordination polyhedron growth units, the growth mechanism and growth habit of the nanometer Mg,Al-hydrotalcite was deeply investigated.