The microstructure evolution and mechanical properties of indirect-extruded Mg-8Sn-1Al-1Zn have been investigated by OM, SEM, XRD, EBSD, TEM and a stand universal testing machine. The results indicate that the high strength Mg-8Sn-1Al-1Zn alloy with ultrafine grained structure can be successfully fabricated by indirect extrusion at 250 oC. During the extrusion process, most of the coarse grains were changed into fine equiaxed grains with average sizes of 1.92 μm. In addition, some nano-sized particles were observed by TEM in the alloy. The extruded alloy reveals a type of fiber texture in which basal poles are preferentially perpendicular to the ED. The formation of the ultra-fine grained structure was ascribed to the dynamic recrystallization and second phase. The tensile yield strength and compressive yield strength of the extruded alloys are 285 and 260 MPa, respectively. The R value of the alloy equals to 0.91. The superior strength and low yield asymmetry are mainly related to the fine-grained structure and dispersive fine second phase particles