With a purpose of developing light-weight materials of high specific strength, particulate strengthened magnesium composites were synthesized by applying reaction milling to magnesium-oxide systems. Powder mixtures of Mg and metal oxides were mechanically milled in a planetary ball mill so that solid state reactions were promoted due to increased contact area between the reactants, decreased diffusion distances and increased diffusion rate. Milled powders were consolidated to the P/M composites by vacuum hot pressing and hot extrusion, and their structures and properties were examined. Changes of constituent phases accompanying with the progress of solid state reactions were studied mainly by X-ray diffraction. In case that the added metal oxides are thermodynamically less stable than MgO, the displacement reaction of oxygen occurred during mechanical milling or subsequent heat treatment, resulting in formation of MgO. The oxides completely decomposed in the Mg matrix include PbO, SnO2, Sb2O3, SiO2, MnO2 and Fe2O3. Partial decomposition was observed for TiO2 and ZrO2 whereas no decomposition occurred for Y2O3 and Sc2O3. TEM observation revealed fine and uniform dispersion of the reaction products as well as the fine matrix grains less than 500nm in size. In case of the Mg-Al alloy matrix, further increases in strength of the P/M composites were obtained in comparison with the Mg matrix. The highest proof strength of 596MPa was observed for the P/M composite from the Mg-Al-MnO2 in compression test at room temperature. This composite shows high specific proof strength of 305 MPa/Mg/m(3). Thus, Mg-particulate composites of high specific strength were synthesized via powder metallurgy routes by applying thermomechanical processing to Mg-oxide powder mixtures.