Thiazole and benzothiazole-containing benzoxazine monomers (abbreviated as PH-at for thiazole-functional monomer and PH-abt for benzothiazole-functional monomer, respectively) have been synthesized. PH-at is characterized with proton and carbon nuclear magnetic resonance spectroscopy (H-1 NMR, C-13 NMR), and the thermal properties of the polymer are tested with thermogravimetric analysis (TGA). Differential scanning calorimetric (DSC) study of PH-at and PH-abt indicates crosslinking caused by the carbon-carbon double bond of the thiazole ring in addition to the usual polymerization via oxazine reaction. In comparison to PH-abt, PH-at is found to have excellent thermal properties and a lower polymerization temperature. The char yield and glass transition temperature (T-g) of poly(PH-at) are significantly higher than other poly (monofunctional benzoxazine)s derived from non-reactive amines. Due to its heterocyclic rings and cross-linkage, poly (PH-at) exhibits great flame retardant property. Flammability of the polymer is evaluated via predicted LOI and heat release capacity as well as experimentally measured heat release capacity using microscale combustion calorimetry (MCC).