A novel soluble phosphorus-containing bismaleimide (BMI) monomer, bis(3-maleimidophenyl)phenylphosphine oxide (BMIPO), was synthesized by the imidization of bis(3-aminophenyl) phenylphosphine oxide, in which its structural characterization was identified with H-1-NMR, C-13-NMR, and Fourier transform infrared spectra. The BMIPO resin, with five-membered imide rings and high phenyl density, was an excellent flame retardant with a high glass-transition temperature (T-g), onset decomposition temperature, and limited oxygen index. In phosphorus-containing BMI/ epoxy/4,4'-methylene dianiline (DDM)-cured resins, homogeneous products were obtained from all proportions without phase separation. Because of the higher reactivity of BMIPO/DDM relative to that of 4,4'-bismaleimidodiphenylmethane (BMIM)/DDM, the increase in the BMIPO/BMIM ratio in this blending resin increased the recrosslinking hazards of the postcuring stage and so lowered the T-g value and thermal stability. The thermal stability of the BMI/epoxy-cured system was lower than that of the epoxy-cured system because of the introduction of a phosphide group into BMIPO, whereas for the Tg value and flame retardancy, the former was significantly higher than the latter: the higher the BMIPO content in the blend, the higher the flame retardancy.