Abstract: Objective This study aims to introduce rare earth element La and element B by adding LaB₆ to improve microstructure and properties of TC4 fabricated by laser powder bed fusion (LPBF), thereby meeting the high-performance requirements of aerospace components. Methods Through systematic investigation of the effect of LaB₆ on microstructure morphology and phase composition of LPBF TC4, underlying mechanism governing its influence on mechanical properties is elucidated. Results The results indicate that during LPBF solidification, LaB₆ acts as a heterogeneous nucleation site, while the in-situ formed nano-La₂O₃ particles pin the β grain boundaries. This combined effect effectively suppresses grain growth both during the LPBF process and subsequent annealing treatment, resulting in nearly equiaxed grains. The addition of LaB₆ leads to the in-situ formation of uniformly dispersed La₂O₃ and TiB nanoparticles, which consume O element within the TC4 matrix. The synergistic effects of grain refinement and nanoparticle strengthening, induced by LaB₆ addition, simultaneously enhance tensile strength and elongation of TC4. Conclusion With the addition of LaB₆, ultimate tensile strength of LPBFed TC4 specimen increases from 1 078 MPa to 1 106 MPa, and elongation rises from 9.5% to 12%.