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　degrees　C.　The　results　show　that　the　alloys　with　x　＜=　0.62,　precipitate　equilibrium　beta-phases　around　the　as-cast　secondary　phases　which　remain　stable　at　400　degrees　C,　while　for　those　with　x　＞=　1.06,　the　precipitation　of　beta-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.