In order to develop ultrahigh strength maraging steels of over 280 kg/mm², effects of Mo, Ti, and Al contents and thermomechanical treatments on the strength, ductility, and toughness of Fe-16-17Ni-15Co-4-7Mo-1.0-4.5Ti-0.05-2.0Al alloys have been investigated. The peak-aged hardness increased in proportion to the amount of strengthening elements shown as (Mo+2Ti+1.75Al) %. The increases in the amounts of those elements, however, led to the unstable fractures in low stress level during tensile tests, and this meant that the strength corresponding to the aged hardness could not be achieved. This tendency was more pronounced in the alloys with higher Al and Ti contents than in the alloys with higher Mo contents. The combination of the superior K_IC and fine grain size can prevent the low stress unstable fracture up to higher strength levels. It has been shown that, by the combination of appropriate alloy compositions and thermomechanical treatments, the ultrahigh tensile strength up to 330 kg/mm² with the excellent ductility and toughness could be achieved in maraging steel.