The interface between matrix and casting tungsten carbide particle is produced by the the particles’ decomposition. Casting tungsten carbide particle is a kind of eutectic product，which is composed of two phases: WC and W2C. The two phases have differing chemical and physical properties, and thus follow different paths to achieve their decomposition process. By observing the decomposition of the particle, a hypothesis was put forward about how the casting tungsten carbide particle decomposes in steel/iron composites. An experiment was then designed to prove this hypothesis. The experimental result shows that heat gain plays a significant role in the decomposition process of casting tungsten carbide particles.