Modification of primary silicon was observed in a hypereutectic Al-Si composite reinforced with TiB2 particles. The primary silicon was found to be refined significantly by TiB2 particles, with the size being reduced from about 25 to 5 mu m. Close observation of the microstructure indicates that the TiB2 particles engulfed by the primary silicon acts as the nucleation sites and maintained consistent orientation establishing in primary silicon. According to the experimental results, it was supposed that a few atomic layer of TiSi2 film between the TiB2 and Si would provide good nucleation conditions. According to the critical values for the interatomic spacing misfit, 10 %, along the matching directions and the d-value mismatch, 6 %, between matching planes, the possibility of nucleation is determined by the conventional plane-on-plane model. Furthermore, the edge-to-edge model is used to predict the matching combinations among Si, C54-TiSi2, and TiB2 in detail.