Abstract: Fe-Ni-Al maraging steels without La and with 0.02% (mass fraction) La were smelted and treated by solid solution at 900 ℃ for 0.5 h and isothermal aging at 500 ℃ for different times (0, 0.25, 0.50, 1.00, 2.00, 3.00, 4.00 h). The effects of La on the evolution of martensitic structure, precipitated phase and tensile properties of the test steel were studied. The results show that the martensitic structure in the test steel had a recovery phenomenon with the extension of aging time, the precipitation of (Nb, Mo) C was coarsened and aggregated. The addition of La could promote the recovery of martensitic structure and the precipitation of carbide, refine the size of carbide and inhibit its aggregation at the grain boundary. By extending aging time, the tensile strength of the test steel without La increased and the percentage elongation after fracture decreased, while the tensile strength and percentage elongation after fracture of the steel with La all increased. The strengthening effect of a large amount of precipitation of (Nb, Mo)C and the promotion effect of La addition on the recovery of martensite structure were the reason for the simultaneous improvement of strength and plasticity of the test steel containg La.