The mechanical property of magnesium alloy depends on the alloy elements and microstructure strongly. The multiple dependency relations are studied by investigating magnesium alloys with different Al and Zn contents. The semi-continuous casting billets exhibit quite different microstructural morphologies between the centre and edge of the cross section. The centre shows fine and uniform microstructure while the edge is coarse and reticular. However the difference is almost eliminated when the billets were homogenized at 380°C for 15h. The generation mechanism of casting microstructure is also discussed in this paper. The results show that the morphology of second phase and the size of grain have greater effect on the mechanical properties than the element Zn. The alloys with uniform, fine and non-dendrite microstructure exhibit both high strength and elongation when Al content is about 6% and Zn content is various from 0 to 3%. In contrast, the strength increases and elongation decreases significantly as Al content increases from 0 to 6%.