The benzoxazine monomer is used to simplify the study of the benzoxazine initiation mechanism. The HPLC retention time and the H-1 NMR spectra of crude products from the benzoxazine reaction are compared with the results from a vast number of pure model compounds, which are synthesized based on the hypothesized mechanisms. Products involved in the process are identified, with species having benzoxazine structures, Mannich base and other components (acetal, nonacetal phenoxy structures, and methylene bridge structure). Initiation mechanisms of benzoxazine, the oxygen protonation and the nitrogen protonation, are proposed.