The crystallization of magnesium hydroxide [Mg(OH)2] from the aqueous solutions of magnesium chloride (MgCl2) or magnesium dodecyl sulfate [Mg(DS)2] was conducted by means of the slowly diffusing of ammonia gas at 30 1C. When the aqueous solution of 5.0×10-3 mol/L MgCl2 was used as a reaction medium, the sunflower-shaped superstructures of thin and branched Mg(OH)2 sheets were obtained at 2 d; the prolongation of reaction time and the increase of MgCl2 concentration caused the formation and the consequent agglomeration of Mg(OH)2 platelets. Interestingly, the aqueous solution of 5.0×10-3 mol/L Mg(DS)2 could be referred as the counterpart of 5.0×10-2 mol/L MgCl2 at the surface nucleation of Mg(OH)2. The precipitates sampled from Mg(DS)2 solutions at 2 or 3 d presented the lamellar aggregates of Mg(OH)2 platelets; while these sampled at 6 and 12 d showed the column-shaped structures of Mg(OH)2 under the transmission electron microscopy (TEM) observation, suggesting the further growth of Mg(OH)2 platelets along the c-axis perpendicular to the [0 01] face. Meanwhile, the nucleation, the crystal orientation, and the crystallographic defect were also discussed, indicating the different crystallization habits of Mg(OH)2 in the aqueous solutions of different magnesium sources.