Abstract: In situ autogenerated Mg₂Si particle reinforced aluminum matrix composites were prepared by using semi-solid extrusion. The microstruc ture and the effects of specific pressure on the mechanical properties of semi-solid extrusion composites were investigated. The results show that, the Mg₂Si reinforcing phases and α-Al matrix in the semi-solid extruded Mg₂Si/Al composites with specific pressures of 255 MPa, 340 MPa and 425 MPa are both spheroidized, but the particle size of Mg₂Si reinforced phase does not change significantly compared with that of casting composite. Tensile test results show that, the semi-solid extrusion composites under the conditions of 255 MPa, 340 MPa and 425 MPa have better mechanical properties than that of the casting composite, and the ultimate tensile strengths and elongations enhance by 71.88%, 53.13%, 43.75% and 378%, 139%, 106%, respectively. The composite with specific pressure of 255 MPa shows the best tensile strength and elongation.