The formations of Co-based Co–B–Si–Ta bulk metallic glasses, as well as their mechanical and corrosion properties have been explored in this work. The alloy compositions were designed by using the cluster line approach and the alloying strategy, where the cluster line referred to a straight composition line linking a specific binary cluster to the third element in a ternary alloy phase diagram. The base ternary compositions in the Co–B–Si ternary system were first determined by the intersection points of two cluster lines, Co–B cluster to Si and Co–Si cluster to B. Then these basic ternary compositions were a small amount of Ta alloyed to further improve the glass-forming abilities. Experimental results revealed that good glass formers with 4mm in diameter occurred at Co 62.2 B 26.9 Si 6.9 Ta 4 , obtained by Ta alloying of the base intersection compositions of Co 7 B 3 –Si with Co 9 Si–B. This bulk metallic glass have not only high thermal stabilities with high T g and T x values, but also exhibit high fracture strength of about 4760MPa, high Vickers hardness of about 1460, a Young's modulus of 198GPa, and good corrosion resistances in 1mol/L HCl and 3 mass% NaCl solutions.