Aluminum-boron carbide particle reinforced composite is an advanced material which can be used in applications such as neutron-shielding components, aircraft, and aerospace structures. In the microstructural characterization of an Al-7%Si-10%B4C die casting, attention is particularly focused on particle distribution and interface reaction products between B4C particles and the aluminum matrix. The quantitative analysis results show that, in a cross-section of the cast part, more particles concentrate in the center and fewer particles are present in the wall regions. Moreover, some particle segregation bands have been observed. The mechanisms of the particle migration are proposed to describe the phenomenon. However, the average particle fraction in any cross-section of the cast part is almost the same. A barrier layer consisting of several sublayers was detected on the surface of B4C particles. Using electron diffraction in selected areas, it is found that these sublayers are composed of Al3BC crystals, TiB2 crystals, Si crystals, and coarse stick-shaped TiB2 particles. In addition, it is observed that Si plays an important role in the formation of a dense barrier layer. The barrier layer can limit B4C decomposition and improve B4C stability in the aluminum melt.