A wide supercooled liquid region exceeding 50 K before crystallization was observed for melt-spun Ti₅₀Ni_25−xCu₂₅Sn_x (x=3 and 5 at%) amorphous alloys. The temperature interval of the supercooled liquid region defined by the difference between crystallization temperature (T_x) and glass transition temperature (T_g), ΔT_x(=T_x−T_g) is 40 K for the 0%Sn alloy and increases to 50 K for the 3%Sn alloy and 60 K for the 5%Sn alloy. With increasing Sn content, the ΔT_x value decreases significantly. A similar increase in thermal stability of the supercooled liquid was also recognized for the 3 at%Sb-containing alloy in the Ti₅₀Ni_25−xCu₂₅Sb_x system. The replacement by 3 to 5 at%Sn for Ni also induces an increase of mechanical strength, in addition to the increase in ΔT_x. The tensile fracture strength (σ_f), Young’s modulus (E) and Vickers hardness (H_v) increase from 1800 MPa, 93 GPa and 530, respectively, for the Ti₅₀Ni₂₅Cu₂₅ alloy to 2050 MPa, 102 GPa and 650, respectively, for the Ti₅₀Ni₂₀Cu₂₅Sn₅ alloy. The crystallization takes place through a single exothermic reaction, accompanying the simultaneous precipitation of multiple (CuTi+Cu₄Ti₃+NiTi) phases. The crystallization mode requires long-range atomic rearrangements for precipitation of the crystalline phases, leading to the increase in the stability of supercooled liquid against crystallization. The high thermal stability of the supercooled liquid enabled the production of bulk amorphous alloys in the diameter range up to about 6 mm by copper mold casting. There is no appreciable difference in the stability of the supercooled liquid region between the melt-spun and cast bulk amorphous alloys. The first synthesis of the Ti-based amorphous alloys with high glass-forming ability and good mechanical properties allows us to expect the future development of bulk amorphous alloys as a new type of high specific strength material.