The relationships between the morphologies of primary silicon and undissolved silicon particles in melt of Al-Si hypereutectic alloys are studied by holding the melt from 600 to 1100 degrees C and then quench interrupting. The variation of growth process and mechanism of primary silicon is also investigated. The results show that undissolved silicon particles in the melt would become the nuclei of primary silicon precipitated in solidification and there is a close relationship between the shape of primary silicon and undissolved silicon particles. The growth of silicon would follow not only twin plane re-entrant edge (TPRE), but also layer mechanism as well. Meanwhile, the shape of primary silicon also relied on kinetic surroundings, such as solute transportation. At higher overheating temperatures of the melt, the primary silicon takes star-like and tree-like shapes.