The fullerene (C₆₀) nanowires, which possess a highly unusual morphology featured by a prismlike central coreand three nanobelt-like wings joined along the growth direction to give an overall Y-shaped cross section, were studied. The experimental observation coupled with theoretical calculation enabled us to elucidate the role of both the fullerene and the solvent in the crystallization process, thus opening up an opportunity for the in-depth understanding of the crystal growth mechanism. More generally, the method developed in this work could be extended to understanding the growth of other inorganic nanowires that have both host and guest molecules involved in their crystal lattices.