Bulk glassy alloy rods with a diameter of 20 mm were produced for Zr₆₁Ti₂Nb₂Al_7.5Ni₁₀Cu_17.5 and Zr₆₀Ti₂Nb₂Al_7.5Ni₁₀Cu_18.5 by a tilt casting method. The replacement of Zr by a small amount of Ti and Nb caused a distinct increase in the maximum diameter from 16 mm for Zr₆₅Al_7.5Ni₁₀Cu_17.5 to 20 mm, accompanying the decrease in liquidus temperature and the increase in reduced glass transition temperature. The primary precipitation phase from supercooled liquid also shows a distinct change, i.e., from coexistent Zr₂Cu, Zr₂Ni and Zr₆NiAl₂ phases for the 65%Zr alloy to an icosahedral phase for the 61%Zr and 60%Zr alloys. These results allow us to presume that the enhancement of the glass-forming ability is due to an increase in the stability of supercooled liquid against crystallization caused by the development of icosahedral short-range ordered atomic configurations. The 60%Zr specimens taken from the central and near-surface regions in the transverse cross section at the site which is 15 mm away from the bottom surface of the cast glassy rod with a diameter of 20 mm exhibit good mechanical properties under a compressive deformation mode, i.e., Young’s modulus of 81 GPa, large elastic strain of 0.02, high yield strength of 1610 MPa and distinct plastic strain of 0.012. Besides, a number of shear bands are observed along the maximum shear stress plane on the peripheral surface near the final fracture site. The finding of producing the large scale Zr-based bulk glassy alloys exhibiting reliable mechanical properties is encouraging for future advancement of bulk glassy alloys as a new type of functional material.