Abstract: Production of anhydrous magnesium chloride by dehydration of bischofite is one of the best ways to utilize the magnesium resource of salt lakes. Formation of magnesium chloride hexammoniate (MgCl₂&#8226;6NH₃) is the critical step of the coupled reaction crystallization process， which is an attractive dehydration process. Primary nucleation of MgCl₂&#8226;6NH₃ by reaction crystallization process was performed in this research. The induction period of MgCl₂&#8226;6NH₃ primary nucleation was measured by the laser method, and classical theory of primary nucleation was used to quantify the nucleation process. When supersaturation ratio (S=C/C*)S>2,homogeneous nucleation occurred; when S<2,heterogeneous nucleation occurred.The relationships among critical free energy of nucleation,critical size of nucleus and supersaturation was obtained.Critical free energy and critical size of nucleus decreased with increasing supersaturation.