A novel macromolecular silicon-containing intumescent flame retardants (Si-IFR) was synthesized, and its structure was a caged bicyclic macromolecule containing phosphorus-silicon characterized by IR. Epoxy resins (EP) were modified with Si-IFR to get the flame retardant EP, whose flammability and burning behavior were characterized by UL 94 and limiting oxygen index (LOI). Twenty percentage of weight of Si-IFR was doped into EP to get 27.5% of LOI and UL 94 V-0. The degradation behavior of the flame retardant EP was studied by thermogravimetry, differential thermogravimetry, scanning electron microscopy, and X-ray photoelectron spectroscopy analysis. The experimental results exhibited that when EP/Si-IFR was heated, the phosphorus-containing groups firstly decompose to hydrate the char source-containing groups to form a continuous and protective carbonaceous char, which changed into heat-resistant swollen char by gaseous products from the nitrogen-containing groups. Meanwhile, SiO2 reacts with phosphate to yield silicophosphate, which stabilizes the swollen char. The barrier properties and thermal stability of the swollen char are most effective in resisting the transport of heat and mass to improve the flame retardancy and thermal stability of EP. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013