The evolution rules of transitional phases β1′ and β2′ during single and double aging and the heterogeneous nucleation of β1′ rods on α-Mn particles in a Mg-6 wt%Zn-1wt%Mn (ZM61) alloy were studied by TEM technique. Age-hardening curves (130, 160, 180, 200 and 230 °C) show that double aging can more greatly enhance hardening level and rate than single aging. The rapid nucleation and the ... [Show full abstract] elongation of β1′ rods occupies the stage before peak aging; the volume fraction increase of β1′ rods results in hardness rise and also a huge accumulation of strain energy; in the over-aging stage the growth of β2′ discs at the cost of β1′ rods can effectively relieve such a strain energy accumulation. G. P. zones of about 10 nm in diameter are densely distributed in matrix after the pre-aging (90 °C/24 h), which can act as the heterogeneous nuclei for β1′ rods and thus considerably raise their number density. Moreover, β1′ rods frequently heterogeneously nucleate on α-Mn particles, because such heterogeneous nucleation can result in a coherent β1′/α-Mn interface and thus reduce the interfacial energy.