In pursuing the new polymers that could provide high mechanical properties and good thermal stabilities, a series of naphthoxazines are synthesized from different hydroxynaphthalenes with aniline and formaldehyde. The molecular structures are confirmed by NMR spectroscopy. After being polymerized in an autoclave, the naphthoxazine derived from 1,5-dihydroxynaphthalene is successfully cured to form the void-free resin. The density and tensile properties of these polynaphthoxazines are measured. Dynamic mechanical tests are performed to determine the T-g, crosslink density, and the activation enthalpy of the glass transition process for the polynaphthoxazines postcured in air at different temperatures. The effect of postcure temperature on the T(g)s of the polynaphthzoxazines is investigated and discussed in terms of crosslink density. The polynaphthoxazine shows a T-g higher than the cure temperature. Fourier transform IR spectroscopy is applied for the molecular characterization of the curing systems. Thermal properties of these polynaphthzoxazines are studied in terms of the weight loss after isothermal aging in static air, the decomposition temperature from thermogravimetric analysis, and the change of dynamic storage moduli at high temperatures.