Laser powder bed fusion (LPBF) is highly suitable for forming 18Ni300 mold steel due to its excellent ability to manufacture complex shapes and outstanding capacity for regulating the microstructure, and it is widely used in fields such as injection molding, die casting and stamping dies. With the increasingly extensive application of mold steel, higher requirements have been put forward for its tensile and wear properties. Adding reinforcing particles into steel is an effective means to improve its performance. In this paper, the Nb/18Ni300 composites were successfully fabricated by LPBF using two kinds of Nb powders with different particle sizes, and their microstructure and properties were studied. The results show that the unmelted Nb particles are uniformly distributed in the 18Ni300 matrix and the grains are refined, especially for the fine Nb particles. In addition, element diffusion has occurred between the particles and the matrix. The main phases of the based alloy are α-Fe and a small amount of γ-Fe. With the addition of Nb, part of the α-Fe is transformed into γ-Fe, and the Nb phase appears. The addition of Nb also improves the hardness and wear property of the composites but slightly reduces the tensile property. After the aging treatment, the molten pools and grain boundaries become blurred, the grains are further refined, and the interface around the particles also becomes thinner. The aging treatment also promotes the formation of reverted austenite. The hardness, ultimate tensile strength and volumetric wear rate of the based alloy reach 51.9 HRC, 1704 MPa and 17.8×10-6 mm3/N.m respectively. In contrast, the sample with fine Nb particles added have the greatest hardness (56.1 HRC), ultimate tensile strength (1892MPa) and yield strength (1842MPa), and the volume wear rate of the sample with coarse Nb particles added has reduced by 90% (1.7×10-6 mm3/N.m).