A novel oligomeric phosphorous-nitrogen containing intumescent flame retardant, poly (4,4-diamino diphenyl methane-O-bicycli pentaerythritol phosphate-phosphate) (PDBPP), was synthesized and characterized. Thermal stability and flammability properties of polypropylene (PP)/PDBPP composites with various PDBPP loading were investigated by thermogravimetric analysis (TGA), limited oxygen index (LOI), and cone calorimeter, respectively. The results showed that the incorporation of PDBPP could improve both the thermal stability and flame retardancy of PP considerably. PP/30%PDBPP system had a LOI value of 28 and its peak heat release rate was reduced by 60% relative to pure PP. Infrared spectrum and field emission scanning electron microscope measurements revealed that PDBPP and PP/PDBPP composites would form a continuous multicellular char layer containing phosphoric acid when exposed to elevated temperature. It was suggested that the very char layer was responsible for the enhanced thermal stability and improved flame retardancy. POLYM. ENG. SCI., 49:1326-1331, 2009. (C) 2009 Society of Plastics Engineers