Intermetallic phases including precipitate constituents in both as-cast and peak-aged Mg–8Zn–6Al-1Sm (wt%) alloys were investigated using transmission electron microscopy. In the as-cast sample, grain boundary intermetallic phases are mainly Mg44Al15Zn41 icosahedral (i) phase, Mg5Zn2Al2 (φ) phase, Mg32(Al,Zn)49 (τ) phase, and Al2Sm phase. During aging treatment, a possible phase transformation from τ to i might occur. Additionally, precipitate constituents in α-Mg matrix of the peak-aged sample are mainly i phase, MgZnAl cubic phase, MgZn2 phase, Mg4Zn7 phase, and aperiodic structures. Two or more orientation relationships between i (or Mg4Zn7, or MgZnAl cubic phase) and Mg matrix were revealed, and ordering planar faults were observed for some MgZn2 precipitates. Finally, calculations indicate that the strengthening effect of Mg–Zn–Al precipitates is more efficient than that of Mg–Zn precipitates, and the high strength of the studied alloy sample is mainly attributed to the semi-continuous intermetallic skeletons and the Mg–Zn–Al based precipitates.