SEM-BSE, TEM and EDS techniques were applied to investigate the effect of Zn content on formation of precipitate phases and long-period stacking structure in MgGd3Nd0.3Znx alloys (x=0-1.25, at%) during aging at 400 oC. The results show that the alloys with x≤0.62 precipitate equilibrium β-phases around the as-cast secondary phases which remain stable at 400 oC, while for those with x≥1.06, the precipitation of β-phases does not take place in the Mg matrix. In the high Zn-content alloys, the as-cast secondary phases are no longer stable; they turn to decompose, and this leads to the formation of Zn- and RE-rich transition regions containing both stacking faults of high density and LPS between those undecomposed secondary phases and Mg matrix. With the aid of the compositional analysis of secondary phases, transition regions and Mg matrix in heat-treated MgGd3Nd0.3Znx alloys, the formation reason and process of the LPS in the high Zn-content alloys were discussed.