Magnesium usually has thermally stable oxide films on its surface, which can be broken by the mechanical force. The hardening phase of Mg_2Si is successfully synthesized by the solid-state reaction between Mg and SiO_2 powder on the route of hot forging and hot extrusion. The result indicates that hot extrusion is benefit to obtain fine and homogenous Mg_2Si/MgO dispersions in the Mg matrix. Comparing with that performed by hot forging, the Mg_2Si/MgO/Mg composite by hot extrusion shows superior mechanical properties.