Abstract: The particle size distribution of feedstock powders critically governs the densification behavior and defect evolution during selective laser melting (SLM). However, its role in NiFeMo alloys remains unexplored. Herein, three powders with single size ranges of 0-20 μm, 20-50 μm, and 30-70 μm, together with seven powder blends, were designed to systematically investigate the influence of particle size distribution on the microstructure and mechanical properties of SLM fabricated NiFeMo alloy. The results demonstrated that the bimodal blend comprising 0-20 μm and 30-70 μm powders at a 1:1 mass ratio yields the best combined performance: yield strength of 339.23 MPa, ultimate tensile strength of 510.98 MPa, elongation of 13.25%, and Vickers hardness of 331 HV10. Electron backscatter diffraction (EBSD) reveals that this optimal particle size distribution refines the average grain size from 19.05 to 13.07 μm and reduces the mean multiples of uniform density (MUD) from 2.14 to 1.74, indicating a more dispersed crystallographic orientation and a weaker texture intensity. Transmission electron microscopy (TEM) confirms a considerably reduced dislocation density within grain interiors, with only isolated dislocations near grain boundaries and scarce stacking faults. Quantitative strengthening analysis demonstrates that grain refinement contributes approximately 47% of the yield?strength increment, while the remaining improvement arises from solid?solution and dislocation strengthening, offset by negligible porosity?induced weakening in the bimodal case. The synergy between Hall-Petch strengthening and a low defect density enables both high strength and good ductility. This work establishes a particle size distribution oriented design route for NiFeMo alloys, highlighting the threshold particle?size ratio for effective bimodal packing and providing a mechanistic understanding distinct from that of Al/Ti systems.