Ultrasound assisted casting method was applied to preparing B₄C_p/Al composites with the addition of B element by adding KBF₄ into Al-B₄C-K₂TiF₆ system. The effects of B element addition on the microstructure and mechanical properties as well as the reaction mechanism at the interface of composites were investigated. Experimental results showed that B element addition altered the B₄C_p/Al interface, induced more small-sized TiB₂ particles to distribute within the matrix. Therefore, cavitation and acoustic flow effects of the melt were intensified, which led to the breaking of B₄C clusters and finally led to the homogeneous microstructure and better mechanical properties of the composites.