Effect of C particle size on the mechanism of self-propagation high-temperature synthesis (SHS) in the Ni–Ti–C system was investigated. Fine C particle resulted in a traditional mechanism of dissolution-precipitation while coarse C particle made the reaction be controlled by a mechanism of the diffusion of C through the TiCx layer. The whole process can be described: C atoms diffusing through the TiCx layer dissolved into the Ni–Ti liquid and TiC were formed once the liquid became supersaturated. Simultaneously, the heat generated from the TiC formation made the unstable TiCx layer break up. However, with the spread of Ti–Ni liquid, a new TiCx layer was formed again at the interface between spreading liquid and C particle. This process cannot stop until all the C particles are consumed completely.