In the study, isocyanate-functionalized hyperbranched polyurethane (HPUNCO) with a degree of branch of 35.2% was synthesized via an A(2)+B-3 method. This novel HPUNCO was used to fabricate a blend system with hydroxyl groups bearing benzoxazine precursor (BZ(OH)) to investigate its effective on the thermal properties of BZ(OH). The structure of HPUNCO is confirmed by Fourier transform infrared (FTIR) and nuclear magnetic resonance spectroscopy. The curing behavior of the reactive HPUNCO/BZ(OH) blend is investigated via differential scanning calorimetry and FTIR analyses. HPUNCO/BZ(OH) blend indicates two successively broad exotherm peaks centered at 132 and 204 degrees C, respectively, which is obviously different from that of BZ(OH) with one broad peak centered at 206 degrees C. Thermogravimetric analysis results confirmed that the thermal stability was enhanced and the char yield of BZ(OH) was increased from 34.3 to 43.2% by cocuring with 10 wt% HPUNCO. POLYM. ENG. SCI., 55:604-613, 2015. (c) 2014 Society of Plastics Engineers