Microstructures and mechanical properties of the Mg-6Gd-4Y(wt.%) alloys with and without 1% Zn additions have been investigated. The results showed that the as-cast microstructure of the Mg-6Gd-4Y alloy studied consisted of the α-Mg matrix and Mg24(GdY)5 secondary phase. However, the as-cast microstructure of the Zn-containing Mg-6Gd-4Y-1Zn alloy studied consisted of the ?-Mg matrix, Mg24(GdY)5 secondary phase and Mg12Y1Zn1 phase which had a 18R long period staking ordered (18R-LPSO) structure. After extrusion the 14H-LPSO phase had been found in the Zn-containing alloy and distributed between the 18R-LPSO strips in the as extruded microstructure. The formation mechanism of the 14H-LPSO phase is precipitation, and the reaction can be expressed as ‘α-Mg′ → α-Mg + 14H’. Zn content had no obvious effect on the precipitation of β series. Aging conducted(T6 and T5 treatment) on both the Mg-6Gd-4Y alloy and the Mg-6Gd-4Y-1Zn alloy caused the formation of β" precipitates. The T6-aging Mg-6Gd-4Y-1Zn alloy exhibited high tensile strength combined with good ductility, and the values of the yield strength (YS), ultimate tensile strength (UTS) and elongation were 309MPa, 438MPa and 6.8%, respectively. It was due to the coexistence of 18R-LPSO phase and small dispersed distribution of the 14H-LPSO phase and the β" precipitates in the microstructure.