The Ti-6Al-4V titanium alloy with YbB₆ addition was fabricated by spark plasma sintering (SPS), and the effects of YbB₆ addition on microstructure and mechanical properties of Ti-6Al-4V titanium alloy were investigated. The results show that with increasing the YbB₆ content, the microstructure of the composites is clearly changed, and the grains are obviously refined. The in-situ formation of TiB whiskers and Yb₂O₃ particles is beneficial to the improvement of mechanical properties of the composites. Furthermore, with the addition of 0.6wt% YbB₆, the relative density, microhardness, yield strength, ultimate tensile strength, and elongation of the sintered alloys are 99.43%, 4030 MPa, 903 MPa, 1148 MPa, and 3.3%, respectively. Compared to those of Ti-6Al-4V alloy, the aforementioned properties of the sintered alloys with 0.6wt% YbB₆ are increased by 0.37%, 13.8%, 38.07%, and 17.14%, respectively. The strengthening mechanism is mainly attributed to the microstructure transformation, grain refinement, and dispersion strengthening. With increasing the YbB₆ content, the fracture mode is the combination of ductile fracture and brittle fracture.