A highly alloyed secondary hardening steel 16NiCo has been studying for more than three years.The optimum alloy compeition of the steel should be 0.16C-2Cr-10Ni-1Mo-14Co. After quenching and ag-ing,the microstructure is composed of highly dislocated lath martensite,a small amounts of interlath retained austenite and twinnig,and some carbides which strengthening the alloy. This stecl possessed excellent combined mechanical properties,the typical mechanical values are as follows:the ultimate strength of 1740MPa,the plane fracture toughness K_IC of 153MPa √mand and the K_ISCC of 82MPa√m. 16NiCo met the requirements of aerospace application,and could be widely used to manufacture the critieal joints,horizontal stabilator shaft,arresting hooks and carrier-based aircraft landing gears,etc.