A novel method was developed to prepare poly(benzoxazinone-imide) by the dealcoholization of poly(amide-imide), having pendent ethoxycarbonyl groups, which was prepared from poly(amide acid). The poly(amide acid) was prepared from the reaction of pyromellitic dianhydride and 4,4Ј-diamino-6-ethoxycarbonyl benzanilide. The curing behavior of the poly(amide acid) was monitored by DSC, which indicated the presence of two broad endotherms, one with maximum at 153 °C due to imide-ring formation and the other with maximum at 359 °C due to benzoxazinone-ring formation. The poly(amide acid) was thermally treated at 300 °C/1 h to get poly(amide-imide) with pendent ester groups, then at 350 °C/2 h to convert into poly(benzoxazinone-imide) by dealcoholization. Viscoelastic measurements of the poly(amide-imide) showed that the storage modulus dropped at about 280 °C with glass-transition temperature (T g ) at about 340 °C. The storage modulus of poly(benzoxazinone-imide), however, was almost constant up to 400 °C and no T g was detected below 400 °C. Also, the tensile modulus and tensile strength of the poly(benzoxazinone-imide) was much higher than that of the poly(amide-imide). The 5% decomposition of poly(benzoxazinone-imide) film was at 535 °C, which reflects its excellent thermal stability. Also, poly(benzoxazinone-imide) showed more hydrolytic stability against alkali in comparison to polyimides.