A novel flame retardant system, ammonium polyphosphate (APP), and expandable graphite (EG) with a new carbon source, poly(diphenolic phenyl phosphate) (poly(DPA-PDCP)), derived from biomass has been proven to be effective in preventing melting drip and improving flame retardancy of acrylonitrilebutadienestyrene copolymer (ABS) in this study, which was manifested by limiting oxygen index (LOI) and vertical flammability (UL-94) tests. The optimal synergy was exhibited at a loading of 30 wt % of three flame retardants in a proper ratio (APP/poly(DPA-PDCP)/EG = 12/3/15). Thermogravimetric analysis result indicated the char residue and the thermal stability could be enhanced because of the synergistic effect of APP/poly(DPA-PDCP)/EG, which is elaborated by a hypothesis of flame retardancy mechanism of the three components. The morphologies of cross-section and char residue by SEM were also described. The dynamic mechanical analysis implied that APP/poly(DPA-PDCP)/EG together can enhance the dynamic mechanical property of ABS. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013