Significant grain refinement was achieved in nano-TiCN/Al-7 wt.% Si composites through growth control induced by nano-TiCN particles (NPs). The microstructures displayed two types of NP distributions: within interdendritic eutectic Si phase and along grain boundaries. This refined microstructure contributed to enhancing mechanical properties. To elucidate the grain refinement mechanism in nano-TiCN/Al–7Si composites, a three-dimensional (3D) sharp interface model (SIM) was developed, integrating the cellular automaton (CA) technique with a transformation matrix comprising three Euler angles. This 3D SIM effectively replicated the dendritic morphology of α-Al. The exact impact of latent heat on grain refinement was clarified. The observed grain refinement induced by NPs could not be numerically reproduced without considering latent heat. The grain refinement in TiCN/Al–7Si composites is governed by multiple factors. As NPs layers inhibits solute diffusion along the solid-liquid interface, the dendrite growth rate reduces, leading to the reduced rate of latent-heat release. Consequently, constitutional undercooling in the surrounding liquid increases, activating more heterogeneous nuclei. The developed SIM model offers deeper insights into the grain refinement mechanism induced by TiCN NPs in Al–7Si alloy.