The effect of the addition of 5 Co, 1 Ti or 1-2 Mo to a Fe-17Ni-15Co-4Mo-2Ti steel which was similar in chemical composition to a commercial 2410 MPa (245 kgf/mm**2) grade 18 Ni maraging steel was studied in order to produce higher strength maraging steels by applying cyclic heat treatment. The effect of alloying elements on microstructures, strength, ductility and toughness was studied by comparing the results with specimens subjected to a conventional solution heat treatment.