We carried out the radical polymerization of methyl methacrylate (MMA) at 0-80 degrees C in the presence of magnesium bromide. The polymerization rate and the molecular weight of the resulting polymers increased according to the amount of the added magnesium bromide as the Lewis acid. It was revealed that the microtacticity of the polymer depended on the amount of magnesium bromide. The effects of the solid surface of magnesium bromide, which is partly soluble in the polymerization systems, were also investigated. The results obtained in this work were compared with the results reported for the polymerization systems in the presence of the other Lewis acids.