The yield asymmetry between compression and tension of magnesium alloy Mg-3Al-1Zn(AZ31) with different grain sizes and textures has been studied by tensile and compressive testing of as-cast,as-extruded and equal channel angular pressed(ECAPed) specimens.The significant yield asymmetry(the ratio of yield strength between compression and tension σyc/σyt is ～0.44) was found in as-extruded specimens and the corresponding microstructure evolution during deformation revealed that {10 ˉ 12} tensile twinning is the underlying reason for the large yield asymmetry.Strong texture and grain size are influential factors for large yield asymmetry.The separate contributions of grain size and texture on yield asymmetry were investigated. The yield asymmetry between compression and tension of magnesium alloy Mg-3Al-1Zn (AZ31) with different grain sizes and textures has been studied by tensile and compressive testing of as-cast, as-extruded and equal channel angular pressed (ECAPed) specimens. The significant yield asymmetry (the ratio of yield strength between compression and tension σyc / σyt is ~ 0.44) was found in as-extruded specimens and the corresponding microstructure evolution during deformation revealed that {10 ˉ 12} tensile twinning is the underlying reason for the Large yield asymmetry. Strong texture and grain size are influential factors for large yield asymmetry. The separate contributions of grain size and texture on yield asymmetry were investigated.