<p>This study investigates the effect of the substitution position of the nitrile group on the curing behavior of benzoxazine and its thermal decomposition behavior in polybenzoxazine. Benzoxazine monomers based on phenol, bisphenol A, and 1,5-dihydroxynaphthalene with nitrile groups introduced at various positions were synthesized, and their structural characteristics were analyzed using ¹H nuclear magnetic resonance (¹H-NMR) spectroscopy. The curing behavior was monitored through differential scanning calorimetry (DSC), which showed that the substitution position affected the curing process. Specifically, when the nitrile group was introduced at the ortho position relative to the amine, the highest curing temperature was observed, followed by the para and meta positions. This can be explained by the electron-withdrawing effect of the nitrile group, which alters the chemical environment and curing behavior. The thermal decomposition behavior was monitored through thermogravimetric analysis (TGA). Compared to polybenzoxazine without a nitrile group, polybenzoxazine with nitrile groups generally exhibited superior thermal stability, with the greatest improvement in thermal properties when the nitrile group was introduced at the ortho position, resulting in about an 11% increase in char yield. However, when the nitrile group was introduced into polybenzoxazine based on 1,5-dihydroxynaphthalene, thermal properties tended to decrease due to the chemical structural characteristics of 1,5-dihydroxynaphthalene. This study emphasizes the importance of the substitution position of the nitrile group in optimizing the thermal stability of polybenzoxazine and provides valuable insights for the design of polybenzoxazines for high-temperature applications.</p>