The precipitates in Mg-Zn-Y alloys in the as-cast state with nominal atomic composition of Mg96Zn1Y3, Mg(96)Zn(2)y(2) and Mg97Zn1Y2 were studied by means of TEM as well as XRD techniques. The results show that there is a phase separation of Zn on the nanometer scale in these alloys. Two kinds of nano-sized precipitates were found, namely MgZn and MgZn2. TEM observation shows that MgZn precipitates are distributed in both the Mg matrix and in Mg24Y5 grains, which is the typical precipitate in Mg-Zn-Y alloys. There is an inherent crystallographic relationship between MgZn and Mg24Y5 precipitates: [110](MgZn)//[112](Mg24y5),(113)(MgZn)//(330)(Mg24Y5). The size of MgZn2 precipitates is much smaller than that of MgZn precipitates. They are distributed only in Mg24Y5 grains, not in the Mg matrix. It is suggested that the nano-sized precipitates, MgZn and MgZn2, can improve the mechanical properties of the Mg-Zn-Y alloys studied. (c) 2006 Springer Science + Business Media, Inc.