Anhydrous magnesium chloride, the dehydration product from magnesium chloride hydrate(MgCl2•nH2O), is a conventional raw material to prepare electrolytic magnesium. Because the dehydration of magnesium chloride hydrate can be accompanied by hydrolysis leading to the production of undesirable hydroxy chloride and oxide compounds of magnesium, it should be carried out in hydrogen chloride gas atmosphere. However, it is difficult to use HCl gas in operation, and the presence of large quantities of corrosive HCl is an obvious disadvantage. Consequently, alternative approaches have been tried for the production of anhydrous magnesium chloride. In this work, the dehydration characteristics of ammonium magnesium chloride hexahydrate (NH4MgCl3•6H2O) were studied, which was considered as an intermediate material for the dehydration of magnesium chloride hydrate by the alternative method for using HCl gas to produce anhydrous magnesium chloride. The results indicated that NH4MgCl3•6H2O decomposed through three steps into MgCl2 below about 290℃, and higher temperatures can cause the magnesium chloride to oxidiz into magnesium oxide. From this work, it was confirmed that dehydration of ammonium magnesium chloride hexahydrate is an effective way for producing anhydrous magnesium chloride even in air atmosphere. It was expected that this process could reduce the production cost and simplify the process of dehydration.