Pure magnesium powders were mechanically alloyed with addition powders of low-melting-metal oxides (SnO₂, PbO and Sb₂O₃) by using a planetary ball mill under an Ar atmosphere. After mechanical alloying, the powders were consolidated to the P/M materials by vacuum hot pressing. Solid state reaction in the mechanically alloyed powders and the P/M materials was studied by XRD and DSC. In these systems, the solid state reaction started to occur during MA processing between Mg and low-melting-metal oxides. The added low-melting metal oxides decomposed and formation of MgO and intermetallic compounds (Mg₂Sn, Mg₂Pb, Mg₃Sb₂) occurred. Such solid state reaction further progressed during hot-pressing and subsequent heat treatment. Microstructures were studied by TEM and mechanical strength was discussed by hardness values for both MA powders and P/M materials.