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The present paper investigates the formation and composition characteristics of Cu-based bulk metallic glasses (BMGs) in the inter-transition metal system Cu-Zr-Ti by using an "e/a-variant criterion" which is relevant to clusters. Three such composition lines, (Cu9/13Zr4/13)(100-x)Ti-x, (Cu0.618Zr0.382)(100-x)Ti-x and (Cu0.56Zr0.44)(100-x)Ti-x, are defined in the Cu-Zr-Ti system. Among them, Cu9Zr4, Cu61.8Zr38.2 and Cu56Zr44 are specific Cu-Zr binary cluster compositions. Alloy compositions are designed along these three composition lines, and alloy rods with diameter of 3mm are prepared by copper mould casting. X-ray and TEM analysis show that BMGs are formed within Ti content range of x = 7.5%-15%, x = 7.5%-12.5% and x = 5%-12% respectively along these three lines. Thermal analysis results further indicate that these BMGs have higher thermodynamic T-g, T-x, T-g/T-l and gamma values, and these values of BMGs on every composition line decrease with increasing Ti content. The optimum BMG composition in this system is Cu64Zr28.5Ti7.5 on the (Cu9/13Zr4/13)(100-x)Ti-x series, which also has the highest hardness and activation energy of crystallization. The characteristic parameters of this BMG are T-g = 736K, T-x = 769K, T-g/T-l = 0.627, gamma = 0.403, H-v = 6.74GPa and Delta E = 3.88 eV, which are all superior to those of the reported BMG Cu60Zr30 Ti-10.