This paper describes the formation mechanism of monophasic Al3BC3 powder with hexagonal plate-like morphology using Al, B4C, and C powders in a stoichiometric ratio. For the synthesis of pure Al3BC3 by the conventional solid-state reaction method, it is important to avoid contamination from the starting powders. Thermogravimetry-differential thermal analysis curves and quantitative analysis by inductively coupled plasma-atomic emission spectroscopy revealed that there was no vaporization of the powder mixture during heating up to 1470 °C in a high-purity argon atmosphere. We found that the phases and Al3BC3 particle morphology after heating at 1800 °C strongly depended on the mixing conditions. In the case of a two-step process (remixing and reheating the sample), particles with random morphology were observed. On the other hand, well-faceted Al3BC3 particles were predominantly formed in a one-step process that involved heating after ball-milling for 24 h.