Phosphorus-containing epoxy-based epoxy-silica hybrid materials with a nanostructure were obtained from bis(3-glycidyloxy)phenylphosphine oxide, diaminodiphenylmethane, and tetraethoxysilane in the presence of the catalyst p-toluenesulfonic acid via an in situ sol-gel process. The silica formed on a nanometer scale in the epoxy resin was characterized with Fourier transform infrared, NMR, and scanning electron microscopy. The glass-transition temperatures of the hybrid epoxy resins increased with the silica content. The nanometer-scale silica showed an enhancement effect of improving the flame-retardant properties of the epoxy resins. The phosphorus-silica synergistic effect on the limited oxygen index (LOI) enhancement was also observed with a high LOI value of 44.5. ; JNL0001549