The 18Ni 300 grade cobalt-free maraging steel T300 was successfully manufactured on an industrial scale by application of vacuum induction melting (VIM) and vacuum arc remelting (VAR) process at Baosteel.With the content of O and N less than 15 ppm and 10 ppm respectively ( 1 ppm =10-6 ),the T300 steel produced by Baosteel obtained a high degree of purity and a good combination of ultra-high strength and fracture toughness.The effects of solution temperature and ageing temperature on tensile property of solution-treated and aged T300 steel were investigated.The results show that the solution temperature slightly influences the strength of the solution-treated steel,indicating that the solution strengthening rather than the grain boundary strengthening is more important to the solution-treated steel.For the solution-treated T300,the elongation and reduction of area dont change much with a rising solution temperature,and remain above 17％ and 70％ respectively at the solution temperature ranging from 800℃ to 1 200℃.For the aged T300 steel,the strength and elongation are reduced with an increasing solution temperature,and the solution temperature has a greater effect on the reduction of area than that on the elongation.The reduction of area of the aged steel reaches a high peak when the solution temperature reaches 920℃.The experimental data indicates that precipitation strengthening makes a contribution of 1 100 MPa to the tensile strength of the aged steel.The ageing temperature exerts a greater effect on the strength than on the elongation and reduction of area.The steel achieves a better combination of strength and toughness at the peak ageing temperature of 500℃.