A large supercooled liquid region exceeding 60 K before crystallization was found in a wide composition range of 15 to 20 at%Cu and 25 to 30 at%Ni for the Ti_94−x−yCu_xNi_ySi₄B₂ amorphous alloys which satisfy the three criteria of multi-components, significant atomic size ratios and negative heats of mixing. The glass transition temperature (T_g) and the reduced glass transition temperature (T_g⁄T_m) of the Ti₅₀Cu₂₀Ni₂₄Si₄B₂ alloy are 742 K and 0.63, respectively. The high thermal stability of the supercooled liquid as well as the high reduced glass transition temperature enabled us to produce bulk amorphous alloys with a diameter of 1 mm by copper mold casting. The crystallization of the Ti₅₀Cu₂₀Ni₂₄Si₄B₂ amorphous alloy takes place through a single exothermic reaction and the four phases of Cu₄Ti₃, CuTi, NiTi and TiB precipitate almost simultaneously. The crystallization mode requires the long-range atomic rearrangements of the constituent elements and the necessity seems to cause the increase in thermal stability of the supercooled liquid. The success of forming the bulk amorphous alloys in the Ti–Cu–Ni–Si–B system with lower densities is important for the future development of bulk amorphous materials with a higher specific strength.