The effects of 0.1wt% boron addition on microstructures and properties of as-cast Ti-6Al-4V alloy were studied. The results show that TiB phase forms in the original β grain boundaries of Ti-6Al-4V alloy due to 0.1% boron addition. Compared with boron-free Ti-6Al-4V alloy, boron added alloy is refined with uniform β grains. In addition, boron addition results in an improved strength, ductility and a changed fracture mechanism. The fracture mechanism of Ti-6Al-4V alloy is as follows: dislocation slip and pile up in the alpha colonies results in stress concentration and then voids growing until cracks forming. However, the fracture of the boron added Ti-6Al-4V alloy is as follows: a stress concentration occurs around the TiB phase and then micro-voids are formed. TiB phase breaks and is separated with the matrix and then cracks nucleate.