A silane coupling agent with a benzoxazine functional group has been synthesized. A series of nonpolar solvents is found to favor benzoxazine monomer formation. Employing these nonpolar solvents, a high-purity benzoxazine functional silane coupling agent is obtained. The effectiveness of this silane coupling agent is evaluated by measuring the interlaminar short-beam shear strength of silane treated glass-fiber-reinforced polybenzoxazine composites.