The effect of boron content on grain refinement of commercially pure aluminum by Al-5Ti-1B was quantitatively assessed. When the boron content is less than 0.03 wt.%, the refining performance of Al-5Ti-1B gradually is weakened as the boron content increases, which is attributed to the reaction of boron with the Al₃Ti interlayer on TiB₂ and the consumption of solute Ti. On the contrary, when the boron content exceeds 0.03 wt.%, the refining performance of Al-5Ti-1B gradually recovers with increasing boron content, which is related to the formation of primary AlB₂ particles that provide additional nucleant substrates.