B4C-Al and B4C-TiB2-Al composites were fabricated by infiltrating aluminum into porous B4C and B4C-TiB2 preforms in vacuum. The effect of TiB2 addition on the microstucture and mechanical properties of the B4C-Al composites were investigated. The flexural strength and the fracture toughness of composite improved greatly as the result of TiB2 addition. The TiB2 addition inhibited the reaction between B4C and Al. The infiltrated aluminum addition was the leading reason for the fracture toughness improvement of the composites. Inter/transgranular fracture mode with many tear ridges and dimples was showing in the fracture surface of the B4C-TiB2-Al composite. The relationships between the microstructures and the mechanical properties of the B4C-TiB2-Al composites were discussed.