Bisphenol A bis(diphenyl phosphate) oligomer (BBO) as flame retardant was synthesized, whose structure was characterized by IR and NMR. In all, 20% weight mixture polyphosphoric acid (APP) and BBO was doped into epoxy resins (EPs) to get 26.0% of limiting oxygen index and UL 94 V-0. The degradation behavior of EP-containing BBO/APP was studied by thermogravimetry, differential thermogravimetry, scanning electron microscopy, and cone calorimeter. The activation energies for the decomposition of EP samples are obtained using the method of Kissinger. The experimental results exhibited that for EP-containing BBO/APP, compared with EP, initial decomposition temperature, maximum temperature at the peak position (Tm), and the activation energy for the decomposition are decreased, whereas the maximum weight loss rate (Rmax), char yields, and the inherent thermal stability are increased. Meanwhile, heat release, smoke production, and CO yield and CO2 yield of EP-containing BBO/APP are much decreased compared with those of EP. The thermal degradation mechanism of EP-containing BBO/APP has been proposed. POLYM. ENG. SCI., 2013. © 2013 Society of Plastics Engineers