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-xTix, (Cu０.618Zr０.382)１００-xTix and (Cu０.56Zr０.4４)100-xTix, 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 igher thermodynamic Tg,Tx,Tg/Tl and γ 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-xTix series, which also has the highest hardness and activation energy of crystallization. The characteristic parameters of this BMG are Tg＝736K, Tx＝769K, Tg/Tl=0.627,γ=0.403, Hν= 6.74GPa and ΔE=3.88 eV, which are all superior to those of the reported BMG Cu6０Zr3０Ti１０.