The effects of Zn addition on the glass forming ability (GFA) and mechanical properties are investigated in Mg 65− x Zn x Cu 25 Gd 10 (x=0, 3, 5, 8 at.%) alloys by X-ray diffractometer (XRD), differential scanning calorimeter (DSC) and mechanical property tests. Thermal analysis shows that with the increasing of Zn content from 0 to 8at.%, the supercooled liquid regions of Mg 65− x Zn x Cu 25 Gd 10 alloys decrease from 68K to 31K, indicating the obvious deterioration in their GFA. On the other hand, however, the mechanical property tests demonstrate that the appropriate addition of Zn (5at.%) in Mg 65− x Zn x Cu 25 Gd 10 alloys greatly improves the mechanical strength, increases their compressive fracture strength and Vickers hardness from 648MPa and 257Hv to 754MPa and 286Hv, respectively. In addition, the fracture surface morphologies have been characterized by scanning electron microscopy (SEM) and the dominant fracture behaviors of the as-cast alloys have been explained as well.