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Mg-(9-x)Sn-xAl-1Zn (x=1, 2, 3 and 4 wt.%) alloys were subjected to indirect extrusion, and the microstructure and mechanical properties of the as-extruded Mg-Sn-Al-Zn (TAZ) alloys were investigated. The TAZ 811 alloy exhibited a finer grain structure than the TAZ 541 alloy due to a larger number of Mg 2 Sn particles, which pinned the grain boundaries and prevented growth of recrystallized grains. The TAZ alloys showed an unusual yield asymmetry behavior. The tensioncompression yield asymmetry increased with decreasing average grain size. The TAZ 811 alloy with a small grain size exhibited a larger yield asymmetry than that of the TAZ 541 alloy having a relatively large grain size, which is mainly attributed to the low Al content and large number of second phase particles in the TAZ 811 alloy.