Bulk metallic glasses (BMGs) are a kind of disorder metal materials with high strength, high hardness and large elastic limit. The development of these materials is one of the most attractive topics for advanced metal materials. How to improve the strength of materials is an eternal topic for material scientists. In this paper, we summarized alloy compositions, thermal stability and mechanical properties of typical Co-based BMGs as the best candidates to design ultrahigh strength BMGs due to their excellent mechanical properties. Possible relationships between the mechanical properties and other physical parameters, like elastic moduli, hardness and characteristic temperature in various kinds of BMG systems are investigated. The Young’s modulus, the Vickers hardness or the glass transition temperature shows a linear dependence on the fracture strength. According to the above results, the design method of element constitution for the ultra-strong Co-based BMGs was presented. The metalloid element(s) with the characteristics of strong covalent bond and the transition metal element(s) with high elastic moduli and high melting temperature should be adopted to be great beneficial for the development of ultrahigh strength Co-based BMGs.