A novel benzoxazine monomer containing aldehyde group, 3-phenyl-6-formyl-3, 4-dihydro-2H-1, 3-benzoxazine (Ald-B), was synthesized via the Mannich condensation of formaldehyde, p-hydroxybenzaldehyde and aniline. Its structure was characterized by Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance (1HNMR). The side reaction in the synthesis is discussed. A novel benzoxazine monomer containing aldehyde group, 3-phenyl-6-formyl-3, 4-dihydro-2H- 1, 3-benzoxazine (Ald- B), was synthesized via the Mannich condensation of formaldehyde, p- hydroxybenzaldehyde and aniline. Its structure was characterized by Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance (1HNMR). The side reaction in the synthesis is discussed.