A novel Mg-8Sn-1Al-1Zn alloy developed for high-speed extrusion was successfully extruded at speeds in a range of 2-10 m/min at a temperature of 250 degrees C. The effect of extrusion speed on the microstructure and tensile properties of the extruded alloys was investigated. Grain size, recrystallization fraction and texture were found to be greatly affected by the extrusion speed, resulting in tensile properties showing lower strength and ductility as the extrusion speed increased. The strength and ductility of the extruded alloys are also discussed in terms of the formation of double twins during the tensile test.