Flame-retardant polybenzoxazine foams containing 1% phosphorus were prepared from diphenolic acid-based benzoxazine and 9,10-dihydro-9-oxa-10-(1-hydroxy-1-methylethyl)phosphaphenanthrene-10-oxide. Statistical predictive models were developed to determine the influence of the foaming time (t_f) and foaming temperature (T_f) on the density, compressive modulus, and compressive strength of the foams. Results showed that the density of the foams exhibited great dependence on t_f, whereas both compressive properties were more dependent on T_f and t_f. Additionally, the flammability of the foams was also characterized by the limiting oxygen index. The presence of 9,10-dihydro-9-oxa-10-(1-hydroxy-1-methylethyl)phosphaphenanthrene-10-oxide greatly improved the flame retardancy of the resulting foams.