Amorphous alloys exhibiting a wide supercooled liquid region above 100 K were found to form in a compositional range from 0 to 3%Co, 0 to 15%Ni and 10 to 23%Cu in Zr₆₅Al_7.5Cu_2.5(Co_1−x−yNi_xCu_y)₂₅ system by melt spinning. The temperature span ΔT_x(=T_x−T_g) between glass transition temperature (T_g) and crystallization temperature (T_x) reaches as large as 127 K for Zr₆₅Al_7.5Ni₁₀Cu_17.5. The T_g and hardness (H_v) increase from 622 to 685 K and 426 to 502 with increasing Co content while the T_x decreases from 749 to 690 K, resulting in the decrease of ΔT_x from 127 to 30 K with increasing Co content. The compositional effect on T_g, T_x, ΔT_x and H_v indicates that there is no close relation between the magnitude of the attractive bonding force and the stability of the supercooled liquid. The high stability of the supercooled liquid against the nucleation and growth of a crystalline phase in the limited composition range seems to result from a highly dense random packing structure consisting of atoms with an optimum atomic size ratio and a large negative enthalpy of mixing.