<p>Spin-active materials with sensitive electron spin centers have drawn significant attention in quantum sensing due to their unique quantum characteristics. Herein, we report a molecular spin sensor based on metallofullerene Y₂@C₇₉N for in-situ monitoring of crystallization behavior and phase transitions in aromatic materials with high precision. Temperature-dependent spin resonance signals of Y₂@C₇₉N dissolved in aromatic materials are analyzed using electron paramagnetic resonance (EPR) spectroscopy. Two functional aromatic materials, 1-chloronaphthalene and a liquid crystal material of 5CB, are selected based on their significant crystallization-related technological applications. For Y₂@C₇₉N in 1-chloronaphthalene, a distinct EPR signal transition attributed to the crystallization of 1-chloronaphthalene. For Y₂@C₇₉N in 5CB, three EPR signal transitions correspond to the phase transitions of crystalline 5CB. Theoretical calculations reveal that the sensing mechanism originates from crystallization-induced alignment of fullerene molecular orientation. This work establishes metallofullerene-based spin probes as a powerful analytical tool for detecting the crystallization processes in materials.</p>