Additive manufacturing of Ti6Al4V alloys via laser powder bed fusion (L-PBF) has demonstrated superior tensile strength compared to conventional methods. However, challenges remain in enhancing ductility and tailoring mechanical properties for specific applications. In this work, we show a feasible method to regulate the mechanical properties of additively manufacturing Ti alloys. Ti6Al4V alloys with different Nb content (1, 3, and 10 wt.%) were fabricated through laser powder bed fusion (L-PBF) in situ alloying using the mixture of Ti6Al4V and Nb powders. The powder mixture shows good printability, and dense Ti6Al4V-xNb alloys are obtained. Although the distribution of Nb is highly heterogeneous, no solidification cracks or secondary intermetallics were detected in both the Nb-rich and Nb-lean regions. The microstructure is gradually refined with the increase in Nb addition, mainly due to the heterogeneous nucleation caused by the partially melted Nb particles. The L-PBF-fabricated T6Al4V-xNb alloys are mainly in α’ martensite phase, even with the addition of 10 wt.% Nb, due to the low content of Nb solute in the matrix. The presence of β phase is suggested around the Nb particles, since a small region with graded Nb content is formed around the Nb particles. The ultimate tensile strength increases from 1050 to 1181 MPa with the addition of 3 wt.% Nb, and the total elongation increases slightly from 8.8% to 10.5%. With the addition of 10 wt.% Nb, the total elongation increases largely to 15.6%, while maintaining a high strength of 1135 MPa. Moreover, the elastic modulus decreases from 105 to 80 GPa with the increase in Nb content to 10 wt.%. The results of this work suggest that L-PBF in situ alloying is a promising approach to optimize the mechanical performance of Ti6Al4V alloys.