The processing parameters for the synthesis of Al3BC3 powder were optimized, and the synthetic mechanism was investigated. The mechanical mixing of the raw powders promoted the formation of secondary phases due to mechanical alloying effect and contamination. Nearly X-ray pure Al 3BC3 powder was obtained after the calcination of the raw powder mixture at 1800°C for 2 h in Ar by suppressing the vaporization of aluminum. During calcination, Al4C3 layer was formed at the surface of aluminum powder by the reaction with carbon, which maintained the morphology of the aluminum powder above its melting temperature. The nucleation and growth of Al3BC3 within aluminum melt began to occur at 1000°C, and became the main synthetic mechanism of Al3BC 3 at 1100°C. The Al3BC3 particles synthesized at 1100°C were porous and were composed of fine hexagonal crystals. The main synthetic mechanism of A3BC3 changed into solid-solid reaction above 1100°C, and a gas-solid reaction promoted the densification of the porous Al3BC3 powder above 1340°C.