Fused-ring naphthoxazine monomers are synthesized from 1-naphthols and cycloimines derived from piperidine and pyrrolidine. Their polymerization has been confirmed by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and Fourier transform infrared Spectroscopy (FT-IR) as well as by solution and solid-state H-1 and C-13 nuclear magnetic resonance spectroscopy (NMR). A polymerization reaction similar to the traditional benzoxazine monomers is found with oxazine ring disappearing and hydroxyl group forming by the nonisothermal FT-IR experiment. Fused-ring monomers, studied in this work, polymerize at significantly lower temperatures compared to the ordinary benzoxazine resins.