Magnesium composites with Mg2Si dispersoids, which are known for their excellent mechanical properties, were successfully fabricated via combination of solid-state synthesis and hot extrusion, and obtained materials were estimated after plastic working. Si powder was mixed with Mg alloy (AZ31) powder to combine AZ31-5wt%Si, and cold compacted. After consolidation, green compacts were pre-heated at 520degreesC, which was decided from the DSC experiment. After pre-heating, the green compacts were hot extruded immediately. Extrusion ratio was R=16, 20 and 26. The hot extruded composites were examined by tensile test, relative density and Vickers hardness, and X-ray diffraction (XRD). The microstructure was also observed by using optical microscopy and SEM. At the same time, the hot forged material as the same composition as the hot extruded one was produced and evaluated. As a result, the effect of refinement of Mg2Si Particles via hot extrusion was confirmed. And their relative densities were above 95%. Mechanical properties of the hot extruded composites were also improved and their strength was extremely higher than that of the hot forged material. Especially in, the case of the composites hot-extruded with the highest extrusion ratio in this study (R=26), high UTS, Y.S. and elongation, 282.2MPa, 226.3MPa and 7.3%, respectively, were showed. Thus, it could be concluded that hot extrusion technique is effective to fabricate fine Mg2Si dispersoids and leads to improvement of mechanical properties Of Mg2Si/Mg composite. This tendency was remarkable as extrusion ratio increased.