Abstract: The setting processes and phase compositions of four MgOMg-Cl₂-H₂O systems with mole ratios; 5:1:13, 3:1:11, 5:1:14.39, and 3.75:1:12.25 at different setting ages by using the X-ray Diffraction and Differential Thermal Analysis (DAT) methods are studied. The same results are shown that the system with different mole ratios leads to different setting processes; the 5Mg(OH)₂MgCl₂ 8H₂O phase may be in either stable state or metastable state, it depends on the existance of gel phase or other phases in the system.