The effects of yttrium (Y) addition on microstructures and properties of the as-cast Ti-6Al-4V alloys were studied. The microstructure and mechanical properties were characterized by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and tensile testing. Ti-6Al-4V alloy showed typical Widmanstatten structure. With Y addition, Y2O3 particles formed and primarily distributed along the prior-β grain boundaries, and it induced the formation of special paralleled α/β lamellas on either side of the prior-β grain boundaries, that was the influence of Y2O3 particles on the Ti matrix. While it exhibited alternate α/β lamellas inside the grains of Ti-6Al-4V-0.3Y alloys. Comparing with Ti-6Al-4V, the Y addition improved the strength slightly, both the paralleled α/β lamellas and Y2O3 caused the transformation from cleavage fracture to intergranular fracture and the significant decrease of elongation. Therefore, the variation of the microstructure of Ti matrix caused by Y2O3 also played an important role in the ductility of the alloys.