The effect of Al addition on the precipitation behavior of a binary Mg-Zn alloy was investigated based on thechanges in the morphology, distribution and element concentration of precipitates formed during aging treatment. The as-castMg-6.0 mass%Zn (Mg-6Zn) and Mg-6.0 mass%Zn-3.0 mass%Al (Al-added) were homogenized at 613K for 48h and at 673Kfor 12h; they were then solid solution treated at 673K for 0.5 h and 1 h, respectively. The Mg-6Zn and Al-added alloys wereaged at 403 K and 433K. The peak hardness of the Al-added alloy was higher than that of the Mg-6Zn alloy at each agingtemperature. Rod-like, plate-like, blocky, and lath-like precipitates were observed in the Al-added alloy aged at 433K for230.4ks, although the rod-like and plate-like precipitates were observed in the TEM microstructure of the Mg-6Zn alloy agedat 433K for 360 ks. Moreover, the precipitates in the Al-added alloy were refined and densely distributed compared with thosein the Mg-6Zn alloy. The Cliff-Lorimer plots obtained by the EDS analysis of the rod-like and plate-like phases in theAl-added alloy peak aged at 433K for 230.4ks were examined. It was confirmed that the phases had higher concentrationof solute Al atom than was present in the phases, indicating that the properties of precipitates can be changed by Al addition.