The precipitation behavior of Mg(17)Al(l2) in monolithic and Al2O3 short fiber reinforced Mg-Al-Zn alloys was investigated by optical and transmission electron microscopies and hardness measurements. The maximum hardness was obtained when the long and short axes of the platelet type continuous Mg17Al12 precipitates were about 0.3 ㎛ and 0.04 ㎛, respectively. The area fraction of the discontinuous Mg17Al12 precipitate nodule reached about 0.23. The coarsening behavior of the discontinuous Mg17Al12 precipitate nodule was found to obey the relationship suggested by the Johnson-Mehl-Avrami model. The slope of the Johnson-Mehl-Avrami plot for the Al2O3, short fiber reinforced Mg-Al-Zn alloy was four times larger than that for the monolithic alloy due to the increased number of nucleation sites. i.e. nucleation at the interface between the reinforcing material and the α-Mg matrix as well as at the α-Mg grain boundaries.