Glass forming ability of Ni-free Zr 100−x−y (Cu 5/6 Ag 1/6 ) x Al y (x=38, 40, 42, 44, 46, 48, 50 and 52at.% and y=6, 7, 8 and 9at.%) alloys has been studied. It is found that bulk metallic glasses (BMGs) with a critical diameter at least 20mm can be synthesized for quaternary Zr 100−x−y (Cu 5/6 Ag 1/6 ) x Al y (y=9at.% and x=38at.%, y=8at.% and x=42–46at.%, y=7at.% and x=46–50at.%) alloys using copper mold casting whereas the ternary Zr 48 Cu 45 Al 7 can only be cast into fully BMG with a diameter of 5mm. The Zr 46 (Cu 5/6 Ag 1/6 ) 47 Al 7 BMG alloy also exhibits a plasticity of about 1.3% during compression at ambient temperature together with a yield strain of 2% and a yield strength of 1916MPa. The developed ZrCuAgAl BMG alloys with high glass forming ability might have biomedical applications.