Abstract: A novel in situ reaction synthesis process, reaction technique in melt, was developed. An in situ particle reinforced Al 4.5 Cu/TiB₂ composite was fabricated with low cost TiO₂, H₃BO₃, Na₃AlF₆ powders and Al Cu alloy. TiB₂ particle is very small, about 0.93 μm in size, and it has spheric shape in the solidification microstructure of the composite. The distribution of TiB₂ particle in the matrix is uniform. The interface between TiB₂ particle and the matrix is combining well. The tensile strength, yield strength and elongation of the composite are 416.7 MPa, 316.9 MPa and 3.3%, respectively, when TiB₂ particle contenTin the matrix is 10 vol%.