In this study, a polybenzoxazine, (PPab) based on phenol and 4-aminomethyl benzoate and its composites containing various amounts of 1,4-benzene diboronic acid, (BDBA) were synthesized by stepwise curing. It has been determined that both thermal stability and char yield were increased as the amount of BDBA incorporated was increased. The improvements in thermal characteristics were associated with condensation reactions of B-OH of BDBA and not only with hydroxyl groups of polybenzoxazine but also ester groups on the aniline linkages yielding a highly cross-linked structure. In addition, self-condensation reactions of B-OH groups generated boroxine net-work. Strong evidences for growth of this net-work on benzoate units of the polymer were detected.